• TABLE OF CONTENTS
HIDE
 Front Cover
 Title Page
 Abstracts of papers presented at...
 Experimental epidemiology of rice...
 Experimental epidemiology of rice...
 Movement of individual viruliferous...
 Effect of temperature on the transmission...
 Response of the etiologic agent...
 Greenhouse and field tests of protective...
 Effect of deep penetrant and other...
 Assessment of rice yield loss due...
 Phytopsthological note: Sampaguita...














Title: Journal of Tropical Plant Pathology
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Full Citation
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Permanent Link: http://ufdc.ufl.edu/UF00090520/00006
 Material Information
Title: Journal of Tropical Plant Pathology
Series Title: Journal of Tropical Plant Pathology
Physical Description: Serial
Language: English
Publisher: Philippine Phytopathological Society
Place of Publication: Philippines
Publication Date: 1975
 Record Information
Bibliographic ID: UF00090520
Volume ID: VID00006
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 1624346
electronic_oclc - 54382605
issn - 0115-0804

Table of Contents
    Front Cover
        Front Cover 1
        Front Cover 2
    Title Page
        Title Page 1
        Title Page 2
    Abstracts of papers presented at the twelfth annual meeting of the Philippine phytopathological society, inc., Cebu City, 5-7 may, 1975
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
    Experimental epidemiology of rice tungro disease. I. Effect of some factors of vector (Nephotettix virescens) on disease incidence
        Page 11
        Page 12
        Page 13
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
        Page 19
        Page 20
    Experimental epidemiology of rice tungro disease. II. Effect of virus source on disease incidence
        Page 21
        Page 22
        Page 23
        Page 24
        Page 25
        Page 26
        Page 27
        Page 28
        Page 29
        Page 30
        Page 31
    Movement of individual viruliferous Nephotettix virescens in cages and tungro infection of rice seedlings
        Page 32
        Page 33
        Page 34
        Page 35
        Page 36
        Page 37
        Page 38
        Page 39
        Page 40
        Page 41
        Page 42
        Page 43
        Page 44
        Page 45
    Effect of temperature on the transmission of rice tungro virus by Nephotettix virescens
        Page 46
        Page 47
        Page 48
        Page 49
        Page 50
        Page 51
        Page 52
        Page 53
        Page 54
        Page 55
        Page 56
        Page 57
    Response of the etiologic agent of citrus greening disease in the Philippines to treatment with broad spectrum antibiotics
        Page 58
        Page 59
        Page 60
        Page 61
    Greenhouse and field tests of protective fungicides for the control of coffee rust
        Page 62
        Page 63
        Page 64
        Page 65
        Page 66
        Page 67
        Page 68
        Page 69
        Page 70
        Page 71
    Effect of deep penetrant and other adjuvants as additive to fungicides in the control of rice blast
        Page 72
        Page 73
        Page 74
        Page 75
        Page 76
        Page 77
        Page 78
        Page 79
    Assessment of rice yield loss due to bacterial leaf blight
        Page 80
        Page 81
        Page 82
        Page 83
        Page 84
        Page 85
        Page 86
        Page 87
        Page 88
        Page 89
        Page 90
    Phytopsthological note: Sampaguita yellow ringspot mosaic
        Page 93
        Page 92
Full Text
-



Philippine



Phytopat9 1ogy'


VOLUME 11 JANUARY AND JUNE, 1975 NUMBERS 1 AND 2


CONTENTS

IAbstracts of Papers Presented at the Twelfth Annual Meeting of the Philippine Phyto-
pathological Society, Inc., Cebu City, 5-7 May, 1975 ...... .................. 1
Experimental Epidemiology of Rice Tungro Disease. I. Effect of Some Factors of Vector
(Nephotettix virescens) on Disease Incidence K. C. Ling .................. 11
Experimental Epidemiology of Rice Tungro Disease. II. Effect of Virus Source on Disease
Incidence K. C. Ling ......................................... 21
Movement of Individual ViruliferousNephotettix virescens in Cages and Tungro Infection
of Rice Seedlings K. C. Ling and M. P. Carbonell ....................... 32
Effect of Temperature on the Transmission of Rice Tungro Virus by Nephotettix Vi-
rescens K. C. Ling and E. R. Tiongco............................... 46
Response of the Etiologic Agent of Citrus Greening Disease in the Philippines to Treat-
ment with Broad Spectrum Antibiotics A. L. Martinez ..................... 58
Greenhouse and Field Tests of Protective Fungicides for the Control of Coffee Rust -
R. B. Valdez, J. R. Acedo and C. 0. Dinozo........................... 62
Effect of Deep Penetrant and Other Adjuvants as Additive to Fungicides in the Control
of Rice Blast D. B. Lapis and Nenita L. Opina ........................ 72
Assessment of Rice Yield Loss due to Bacterial Leaf Blight D. B. Lapis and Sawatdee
Liansuthisakon ............................................... 80
,/HYTOPATHOLOGICAL NOTE:
D. A. Benigno ................................................. 91


Official Organ of
THE PHILIPPINE PHYTOPATHOLOGICAL SOCIETY, INC.








THE PHILIPPINE PHYTOPATHOLOGICAL SOCIETY, INC.
Founded 10 October 1962

BOARD OF DIRECTORS 1975-1976

President: A. J. QUIMIO, UPLB, College, Laguna
Vice-President: D. A. BENIGNO, UPLB, College, Laguna
Secretary: LINA L. ILAG, UPLB, College, Laguna
Treasurer: T. T. REYES, UPLB, College, Laguna
Auditor: A. N. PORDESIMO, UPLB, College, Laguna
Board Members: F. C. QUEBRAL, UPLB, College, Laguna
O. R. EXCONDE, UPLB, College, Laguna
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Philippine


Phytopathology

Official Organ of the Philippine Phytopathological Society, Inc.




EDITORIAL BOARD

P. M. HALOS, Editor-in-Chief, Department of Plant Pathology
UPLB. College, Laguna
A. J. QUIMIO, Associate Editor, UPLB, College, Laguna
R. B. VALDEZ, Associate Editor, UPLB, College, Laguna

BUSINESS MANAGEMENT

C. A. BANIQUED, Business Manager, BPI, San Andres, Manila













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ABSTRACTS OF PAPERS PRESENTED AT THE TWE ANNUL ThNG
OF THE PHILIPPINE PHYTOPATHOLOGICAL-!OIETY; INC., CEBU CIT -7
MAY 1975'. Co
,f /


Chemical c
CS. Atienza a
Pyracarboli
temic fungicic
treatment and
cane smut. T
ieties, viz., P1
were used.
The fungic
treatment (pro
at 300 ppm fi
ever, the chei
eradicant and
trations) for
concentrations
lation as erad
further experi

/ Evidences
dy observed o
pines is viral
and Maria Aug
The fanlea
of grapevines,
dinal variety,
mine whether
is viral in natu
To prove t
ease, transmis!
means cuttingn
scope examine;
purified sap
done. Results
revealed that
mechanically
globosa and I
several indicate
fanleaf virus,
materials reve


(Arranged alphabetically acc riding to first author's surname) / .

control of sugarcane smut. \ virus-like particles under' the electron
nd F. R. Husmillo. microscope; bneaSiinng-about 30 nm in
d (pyran derivative), a sys- diameter-and very similar to those re-
de was tested as seedpiece ported by other workers in Europe and
Sfoliar spray against sugar- U.S. In addition to these, cuttings obtain-
wo highly susceptible var- ed from diseased vines also produced
iil 56226 and CAC 57-11, fanleaf-like leaves under greenhouse con-
ditions.
These three different tests are evi-
;ide was effective as dip dences to show that the grapevine fan-
)tectant) against the disease leaf virus (GFV) is already present in the
or both test varieties. How- Philippines.
nical was not effective as
as foliar spray (all concen- Pathogenicity and control of plant
both test varieties. Higher parasitic nematodes associated with
Sof the fungicidal formu- grapes. R. G. Davide and Ruth M.
icant is recommended for Sarra.
nentation. Host-parasite relationships between
to show that fanleaf mala- grape cuttings and the rootknot nema-
'n grapevines in the Philip- tode, Meloidogyne incognita were stu-
in nature. D. A. Benigno died. Three-month old rooted cuttings of
usta Favali. Black Ribier and Red Cardinal varieties
which were used as test plants were as-
if characteristic of leaves sessed three months after nematode in-
particularly the Red Car- oculation. In pot experiments, cuttings
was investigated to deter- inoculated with 10, 20 and 30 egg masses
the said fanleaf symptom all exhibited root gallings with no signifi-
re or not. cant variations between them. However,
he viral nature of the dis- root and top growth were significantly
sion by sap, by vegetative reduced with increased levels of inoculum.
igs), and electron micro- Galling was not observed in the un-
ations of both crude and inoculated controls. Plants whose root
from diseased leaves were systems were inoculated with 30 egg
obtained from these tests masses had less tillers. Uninoculated
an infectious entity was plants had more tillers and more robust
transmitted to Gomphrena root systems.
Phaseolus vulgaris, two of
or plants for the grapevine Soil fumigation experiments were con-
and that said infectious ducted under field conditions in three
ealed numerous isometric different localities. Of the 5 nematicides








Philippine Phytopathology


tested in Nasugbu, Batangas, Vydate, the
most effective there, gave 92.37% and
92.66% control of nematodes 1 and 3
months after fumigation, respectively.
Corresponding values for Temik 10G
which gave the lowest percentage con-
trol were 63.58% and 63.69%. At the
BPI Experiment Station in Mandaue,
nematode counts revealed high per-
centages of control with Furadan 3G,
Fumazone (75 E. C.) and Temik 10G.
HOE 2960-5G gave the lowest percent-
age control of nematodes in this set of
assessments made 2 and 4 months after
application of nematicides. In U.P. Los
Bafios, experimental plots of Red Car-
dinal and Brazilian hybrid varieties were
used. Yield data based on the number
of bunches and their gross weights were
taken for plants in each plot. Plants in
plots fumigated with Vydate, Furadan
3G and Fumazone 75 E.C. yielded higher
than those in non-fumigated plots.

Smut incidence in Bogo-Medellin Mill
District, Cebu. R. V. Estioko and
H. H. Las Pifias.
A survey of sugarcane smut (Ustilago
scitaminea Sydow) in Bogo-Medellin Mill
District, Cebu was made in July to
August, 1974. Eighteen sample farms
with a total effective area of 269 hec-
tares or 3.5 per cent of the mill district
area was surveyed. The survey showed
that Phil 56-226 obtained a higher degree
of infestation than H37-1933 with 15.86
and 8.68 per cent infestation, respective-
ly. In both varieties, the incidence of
smut on ratoon crops was higher than
on plant canes. Three to five-month
old canes were more susceptible than
any age level of sample plants.
Based on the extent of infestation,
the estimated losses due to smut for
Crop Year 1973-74 in the district was
about 30,200 tons or 48,700 piculs sugar.


'Electron microscopic investigations of
viruses attacking economic plants in the
Philippines. Maria Augusta Favali, D.
A. Benigno and M. L. Retuerma.
Most studies on virus diseases of eco-
nomic plants in the Philippines were
on host ranges, physical properties and
transmission. Very little effort was done
to establish the kind of virus or viruses
causing such diseases. This paper reports
the results of preliminary investigations
on the morphology of some plant viruses
infecting a number of economic plants.
Crude as well as purified extracts
obtained from infected leaves were both
investigated for the presence of virus
particles. Samples from these two virus
sources were either stained with 2%
phosphotungstic acid (PTA) or/and
shadow cast with a heavy metal like
chromium.
Results obtained from these investi-
gations revealed three types of virus
particles namely isometric or sphericals,
rigid rods and flexible rods.

Pyracarbolid, a new systemic fungi-
cide for the control of bean rust (Uro-
myces phaseoli). Jules Guerassimoff and
E. B. Akiew.
Pyracarbolid is the approved common
name for 2-methyl-5, 6 dihydro -4-H-
pyran-3-carboxylic acid anilide, a systemic
fungicide of low mammalian toxicity
specific for the control of rust and
smut diseases, and also damping-off dis-
orders caused by Rhizoctonia solani.

Foliage sprays of pyracarbolid at
0.01125% a. i. with a 14-day spray
interval in 'Alno' string beans (Phaseolus
vulgaris) in Benguet have given both
eradicative and preventive control of
heavy infestations of bean rust (Uromyces
phaseoli), the most serious leaf disease
on this vegetable crop. Further studies


Vol. 11








Jan. & June, 1975


are being conducted on string beans,
mung beans and soybeans to confirm
these firidings.

Market diseases of vegetables in the
Philippines. Lina L. nag and L. M. Ilag.
The market diseases that were ob-
served are given and the factors that
contribute to disease production are dis-
cussed.
Seventy-eight (78) out of 95 micro-
bial isolates from various infected vege-
tables in the market were found to be
pathogenic. The pathogens were species
of Rhizopus, Pythium, Phoma, Oospora,
Aspergillus, Fusarium, Penicillium, Col-
letotrichum, Gloeosporium, Phomopsis,
Curvularia and soft rotting bacteria.

Movement of individual viruliferous
Nephotettix virescens in cages and tung-
ro infection of rice seedlings. K. C.
Ling and M. P. Carbonell.
The movement of 1,330 tungro-viruli-
ferous adults of N. virescens was ob-
served from 0800 to 1700 hours after
the insects were individually confined
in screen cages with seedlings of IR8
or Taichung Native 1 (TN1). Only three
kinds of movement, seedling-to-seedling
- from a seedling to another seedling,
off-seedling from a seedling to a
non-seedling site, and back-to-seedling
- from a non-seedling site to a seedling,
that are related to the spread of tungro
disease were studied.
Some insects moved during each ob-
servation period although the percentage
of insects that moved varied from an
hour to another. A higher percentage
of the insects made the seedling-to-seed-
line movement than the other two kinds
ot movement. Also, the number of seed-
ling-to-seedling movements per insect was
higher than that of the other two kinds
of movement.


As compared with the viruliferous
insects on TN1 seedlings, more insects
on IR8 seedlings moved and the insects
on IR8 seedlings made more numbers
of movement in a given period of time.
Consequently, more IR8 seedlings were
visited by the insects but the duration
of the insect's visit per seedling and
the time interval between two move-
ments of the insects became shorter.
Regardless of rice variety, the seedlings
that became infected had a significantly
longer insect's visiting duration than
those that did not become infected.
The results suggest that a reason for
field resistance of a rice. variety may be
related to non-preference of the variety
by the insect vector that results in fre-
quent movement of the insect vector.
Consequently, the variety has shorter
insect's visiting duration per seedling in
the field that results in low percentage
of infection. When the variety is in-
oculated in the greenhouse where the
insect cannot move freely, the infection
increases because of longer insect's visit-
ing duration per seedling.
The highest number of infected seed-
lings obtained was in 9-hour period, that
is about 11 infected seedlings/insect/
day with the assumption that the in-
fectivity of the insect for the first 9
hours remains unchanged for the rest
of the day indicating the limitation of
infective capacity of the insect trans-
mitting the tungro virus. Hence, the
population of the insect in a field could
be a major factor in so far as the out-
break of the disease is concerned.


Effect of temperature on tungro trans-
mission by Nephotettix virescens. K. C
Ling and E. R. Tiongco.
Studies on various aspects of trans-
mission of tungro virus by adults of


ABSTRACTS







Philippine Phytopathology


N. virescens as affected by temperature
were undertaken in the IRRI Phytotron.
At temperatures from 13 to 34 C, the
life span of the tungro-viruliferous insects
was significantly longer at low tempe-
rature than at high temperature. When
the insects were transferred individually
and successively at 30-minute intervals
for 10 hours, the highest infective capa-
city was obtained at 34 C.
The virus was acquired and inoculated
by the insects from as low as 10 C to
as high as 38 C. However, there seemed
to be no striking difference in percentage
of infected seedlings from 25 to 38 C.
When the viruliferous insects were
confined at 7C for 1 or 2 days, the
infectivity of the insects were not lost
considerably or at least at a much lower
rate than the insects kept at room tem-
perature.


Reactions of some abaca varieties to
abaca wilt caused by Fusarium oxys-
porum var. cubense. Lydia V. Magnaye.
Six (6) abaca (Musa textilis Nee)
varieties were tested in the laboratory
for resistance to abaca wilt caused by
Fusarium oxysporum var cubense. Pot-
ted test plants of Libuton, Linawaan.
Itihin-balud, Inosa, Tangongon Lawaan
and Bongolanon Davao were inoculated
with corn meal-sand culture of the fungus
and indexed by ocular inspection within
60 days. Those that did not show exter-
nal symptoms were finally dissected
after 2 months to determine the extent
of rhizome discoloration.
Varieties Inosa and Linawaan appeared
to be significantly most resistant among
the entries: No significant differences
in susceptibility Were observed among
the rest.

Distribution and epidemiology of


Isabela strain of Xanthomonas oryzae.
- S. D. Merca, A. G. dela Rosa, A.
Ronduen and H. E. Kaufman.
The Isabela strain of X. oryzae is
widely distributed in rice field of South-
ern Isabela. Isolates with similar levels of
virulence on IR20 have been isolated
from diseased leaf specimens from Va-
lencia, Bukidnon (variety IR1561),
from Aborlan, Palawan (variety Benzar)
and from San Miguel, Bulacan (variety
IR26). The frequency of the aggressive
strain from these areas was rather low.
Field studies at the Cagayan Valley Ex-
perimental Station, San Mateo have in-
dicated that epidemiologically the Isabela
strain does not cause as much disease
on resistant varieties such as IR20 as
does the typical strains on susceptible va-
rieties such as IR24. Therefore, the more
aggressive strains apparently do not at
the moment pose a serious threat to
bacterial blight resistant varieties which
are widely grown in the Philippines.


Oil as a non-conventional fungicide
and as a carrier of conventional fungi-
cides for the control of rice blast in
the seedbeds. A. B. Molina, Jr. and
D. B. Lapis.
An experiment was conducted to eval-
uate mineral oil as a non-conventional
fungicide and as a carrier of benomyl
and Hoe 17411 (OF) for the control of
rice blast in the seedbeds during the dry
and wet seasons of 1974.
Oil alone significantly reduced the
lesion counts by 45% compared with the
control but it. was not as effective as
benomyl and Hoe 17411 (OF). When
the oil was used as a carrier, it improved
the effectiveness of Hoe 17411 (OF)
but not that of benomyl. This may be
attributed to the solubility of Hoe 17411
(OF) but not of benomyl in oil.


Vol. 11







Jan. & June, 1975


Although actual phytotoxicity ratings
were not made, it was observed that
seedlings treated with oil exhibited a
certain degree of phytotoxic reaction
during the dry season.

Pathogenic races of Pyricularia oryzae
in the Philippines. S. H. Ou, F. L.
Nuque, J. M. Bandong, S. P. Ebron and
T. T. Ebron, Jr.
The pathogenic races of 3,225 isolates
of Pyricularia oryzae in the Philippines
studied during the last 10 years were
determined with the use of 12 Philip-
pine differentials and 8 international dif-
ferentials.
The Philippine differentials can iden-
tify a theoretical number of 4,096 patho-
genic races. Based on the reaction of the
Philippine differentials, the isolates were
classified into 255 pathogenic races. Of
the total number of isolates tested, 26.8.
5.4, 3.7 and 3.2 per cent belong to P8,
P12, P92 and P9, respectively, indicating
that they are common pathogenic races.
Other common races are P11, P15, P16,
P17, P18, P19, P20, P23, P25, P30, P36,
P50, P52, P81, P82, P87, P115, P121,
P136, P201 and P222 with a range of 1 to
2.8 per cent of the total isolates tested.
Ninety five races, for example, P3, P34,
P42 etc. have one isolate each equivalent
to 0.03 per cent of the total isolates.
These races were detected only once and
are considered rare races. Race P8 was
detected annually throughout the period
of study suggesting that it was the most
frequently occurring race. Other races
occurred intermittently or periodically.
Using the virulence index as a criterion,
P150 was considered most virulent for it
infected all the 12 Philippine differentials.
Thirty seven races like P9, P10, P53, etc.
were moderately virulent, capable of in-
fecting six varieties. Races .P26, P220,
P224, P232 and P251 were least virulent


attacking only one variety.
The international differentials can clas-
sify 256 race groups. The isolates deter-
mined by the international differentials
were classified into 78 race groups. Of the
total number of isolates tested, 45.5,
10.4, 6.2, 5.6, 4.9 and 4.2 per cent
belong to IA-109, IA-65, IA-45, IA-110,
ID-13 and ID-14, respectively. Twenty
seven race groups like IA-3, IB-62, IC-8,
etc. have only one isolate each or 0.06
per cent of the total isolates. These race
groups occurred only once during the
period of study and were considered rare
races. Race group IA-109 was present
annually indicating that it is the most
frequently occurring one. The rest of the
race groups occurred periodically.
The resistance of the 12 Philippine
differentials and two other varieties to
the Philippine races was likewise eval-
uated. Pai-kan-tad was resistant to 81 per
cent of the races; Kataktara, 74; CI 5309,
64; Chokoto, 62; Wagwag and Co 25,
60; Raminad Str. 3, 42; Taichung TCWC,
41; Lacrosse, 37; Peta, 35; Sha-tiao-tsao,
31; and KTH-17, 10. Two other varieties,
Carreon and Tetep were resistant to the
96 and 83 per cent of the races, res-
pectively. The results indicated variation
in the resistance of the varieties to the
races.

Epidemiology of rice blast disease.-
S. H. Ou, F L. Nuque and S. P. Ebron.
The study of the epidemiology of the
rice blast had been initiated to understand
the effect of climatic factors on various
disease processes incited by the fungus
and to develop a system to forecast the
disease in the tropics.
The combined factors of temperature
from 21 to 36 C and period of leaf
wetness from 4 to 24 hours in relation
to number of lesions produced by the
same spore concentration of the blast


ABSTRACTS








Philippine Phytopathology


fungus on three-week old rice seedlings
were studied one week after inoculation.
The effects of constant temperatures of
20 to 35 C at 75 per cent relative humidi-
ty and constant relative humidities of
50, 75 and 85 per cent at 26 C on the
colonization of the blast fungus were also
investigated.
The results showed that the optimum
range for infection was between 21 to
28 C, and 21-22 C seemed to be most
favorable. The period of wetness greatly
affected infection by the blast fungus.
At 12 hours period of wetness, apprecia-
ble infection occurred. With longer period
of wetness up to 24 hours, the number
of lesions was increased regardless of
temperature.
At 75 per cent relative humidity, the
temperature range of 20 to 25 C was
more favorable for colonization as well
as for infection. Eighty five per cent
relative humidity was more favorable for
disease development than 50 per cent.

The international uniform blast nur-
series, 1963-1973 results. S. H. Ou,
F L. Nuque and T. T Ebron, Jr.
This report presents the results ob-
tained from 1963 to 1973 from the O0
countries participating in the blast nur-
sery program. A total of 551 sets of
seeds have been distributed to these coun-
tries and 325 test results have been re-
ceived.
The test results were published in the
International Rice Commission Newsletter
every .two years. The tests enabled us to
evaluate varietal resistance to blast more
comprehensively since these varieties were
exposed to numerous prevailing races over
wide geographic regions. The results show-
ed that reactions range from highly re-
sistant to highly susceptible. Varieties
such as Tetep, Nang.chet cuc, Tadukan,
Mamoriaka, Carreon, Ram Tulasi (Sel.),


C46-15, Dissi Hatif and others consistent-
ly showed very broad spectrum of resis-
tance to blast whereas varieties like Kung-
shan-wu-shen-ken and Fanny exhibited
very susceptible reactions in majority of
the tests.
A preliminary report on the effective-
ness of some fungicides against pineapple
disease of sugarcane. Teresita C. San
Pedro and Eleanor Bacong-Rivera.
Five chemicals at four different levels
of concentration were used to test the
effectiveness against pineapple disease of
sugarcane. The chemicals tested were Ben-
late, Vitigran-blue, Carbendazole, Dithane
M-45 and Difolatan 4.
So far, Benlate at the rate of two
grams per gallon of water showed pro-
mising results against the disease, fol-
lowed by Carbendazole and Difolatan
under laboratory and greenhouse condi-
tions. The two other fungicides, Vitigran-
blue and Dithane M-45 were unable to
control the disease.
Preliminary studies of four micro-
organisms against the growth and deve-
lopment of Selerotium rolfsii as a means
of biological control. A. L. Piamonte
and F C. Quebral.
Four soil microorganisms were assayed
against the growth and development of
Sclerotium rolfsii as a means of biological
control. Of the assayed organisms, only
Trichoderma sp. showed promising re-
sults. Agar plate and soil incorporation
tests revealed that Trichoderma greatly
suppressed Sclerotium by growing over
the sclerotial bodies and consequently
inhibiting mycelial growth. Trichoderma
sp. probably lysed Sclerotium rolfsii.
Some of the sclerotia that were seeded
with Trichoderma sp. became very soft
and watery substance leaked when picked
up with the force.
The other organisms namely Rhizopus
sp., Aspergillus niger, A. flavus and Peni-


Vol. 11








Jan. & June, 1975


cillium sp. did not suppress the growth of
Sclerotium in agar plate and soil tests.
Among soils seeded with the different
test microorganisms, Trichoderma sp.
greatly inhibited sclerotial body forma-
tion by Sclerotium rolfsii. This was fol-
lowed in descending degree of inhibition
by Penicillium sp., Rhizopus sp., A. niger
and A. flavus. Confirmatory test through
agar plating showed that out of 28 seeded
sclerotia, on the average, only 9.33 re-
tained viability after being acted upon by
Trichoderma sp. Sclerotia obtained from
soils seeded with the other microor-
ganisms were 100% viable.

Evaluation of selected fungicides for
the control of grape downy mildew. -
A. N. Pordesimo.
Field tests during the wet season
(August-October, 1974) were conducted
at the Grape Experiment Station, Rosario,
Batangas and in two other vineyards in
Sto. Tomas, Batangas. In all 3 locations,
Mancozeb at 240 g in combination with
either Thiodan at 80 ml or Imidan-Ma-
lathion 1-IE at 198 per 100 liters con-
sistently and effectively controlled downy
mildew. Bordeaux mixture 4-4-50 (960
g CuSO4 and 960 g lime in 100 liters) was
just as effective but was rather phytotoxic
to leaves, flowers and young developing
fruits. Captan at 240 g/100 1 water was
ineffective against downy mildew but
showed some beneficial effects on fruit
setting.
The effect of fungicide spray for
downy mildew control during the dry
period (January-March, 1975) was de-
monstrated only in the Grape Experiment
Station at Rosario, Batangas where epi-
phytic conditions prevailed that permitted
a more drastic assay for the test materials.
In Balayan, Batangas where downy
mildew incidence was low throughout the
period of the experiment, the perfor-


mance of the test fungicides was not dis-
cernible. Where downy mildew epiphy-
totics prevailed, Grapefun at 420 g/100
liters excelled the performance of Man-
cozeb (the standard fungicide used in the
tests) at 240 g/100 liters in controlling
downy mildew; this recently marketed
fungicide can be used in alternation with
Mancozeb in spray programs for grape
downy mildew control.
Effect of fungicide and insecticide
sprays on mango fruit setting and post-
harvest rots. A. N. Pordesimo and F. C.
Barredo.
In the field test conducted on 45-year
old trees at Yuson's Farm, Jaen, Nueva
Ecija in November 1971, Mancozeb at
236 g plus Karathane WD at 30 g plus
Thiodan at 30 ml plus surfactant at 30
ml per 100 liters improved fruit setting of
smudged and irrigated Manila Super Man-
go (formerly var. Carabao) to produce
off-season fruits. In a separate trial, the
application of fungicide-insecticide sprays
as pre-bloom, bloom and post-bloom
sprays minimized incidence of anthrac-
nose, Diplodia basal rot and fruit fly on
fruits harvested from sprayed trees.
In the 1973 field trial at Calumpang,
San Miguel, Bulacan, conducted on 8-10-
year old grafted Manila Super Mango
trees, fungicides and insecticides, ap-
plied separately or in combination, as
bloom sprays at weekly intervals im-
proved fruit setting. Mancozeb at 240 g
and copper oxychloride at 480 g/100
liters were superior than Captan at 300 g,
Zineb at 240 g and Dikar (Mancozeb at
210 g plus Karathane WD at 30 g) per
100 liters. Imidan 50 WP at 240 g/100
liters excelled 5 other insecticides in im-
proving fruit setting.
Spray applications at 7-10 day inter-
vals with fungicides in combination with
insecticides during flower initiation, nec-
tar stage, and fruit development did not


ABSTRACTS







Philippine Phytopathology


show distinguishable effect on fruit set-
ting in the 1974 field trials. But the carry-
over protection of 3 spray applications
against post-harvest rots was reflected in
the number of rotten fruits 2 weeks after
harvest under ordinary room conditions;
there was less number of rotted fruits har-
vested from sprayed trees.


A new bacterial disease of cassava in
the Philippines. A. J. Quimio and R. D.
Daquioag.
A bacterial disease of cassava hitherto
unreported from the Philippines was ob-
served in August, 1974 at the Central Ex-
periment Station, University of the Phil-
ippines at Los Bafios, College, Laguna.
The disease is characterized by leaf spot-
ting and blighting, die-back, wilting and
vascular necroses. Leaf spots are at first
water-soaked and dark green, becoming
angular with dark brown to reddish
brown centers and water-soaked dark
green margins with age in old leaves; in
young leaves, the spotting is followed by
blighting and finally premature falling-off
of affected leaves. Premature defoliation
is followed by severe die-back. Wilting,
the most devastating effect of the disease
is commonly observed after the leaf spot
and blight phase of the disease. Vascular
necroses start from the leaf petioles and
scars and may reach the base of the stem.
Longitudinal cracks, gums, and brown
necrotic patches of tissues may develop in
the green stem. The following varieties
were noted susceptible based on field ob-
servations of relative disease reactions;
Green Twig (most susceptible), Aipin
Manteiga, Dapecol, Juice Hawaii Balaba-
gan, Thailand No. 2 and Carmen Singa-
pore. The varieties Cubano, Hawaii No. 2
and Mameya were noted resistant.
Initial results of studies on the etio-
logy of the disease showed that the causal
bacterium belongs to the genus Xantho-


monas. It is suspected that the disease is
similar to cassava blight caused by Xan-
thomonas manihotis which is considered
as one of the major limiting factors of
cassava production in several countries of
South America and Africa.
Systematic look for strong resistance
to bacterial blight in rice. A. G. dela
Rosa, S. D. Merca and H. E. Kauffman.
Cultivars in the germplasm bank at
IRRI are systematically being screened to
identify new sources of resistance to bac-
terial blight. New entries are being screen-
ed each season to add to the more than
25,000 entries which have already been
screened. Genetic studies are being ini-
tiated in some of the 600 cultivars to de-
termine if the source of resistance is dif-
ferent from the 3 already known. The Isa-
bela strain is used to detect those with a
recessive gene for resistance. RP633 from
India progeny lines with the combined
dominant gene (from IR22) and the re-
cessive gene (from BJI) have clearly had
the strongest resistance in the Interna-
tional Bacterial Blight Nursery tested in
20 locations in 10 countries.

Chemical control of Meloidogyne in-
cognita on tomato. Sr. J. S. Ruelo and
R. G. Davide.
Four locally available chemicals name-
ly Hostathion 5G, Furadan 3 G, Temik
10G and Fumazone 75EC were evaluated
for nematicidal action against root-knot
nematode, Meloidogyne incognita. Four
concentrations (1, 10, 100 ppm and re-
commended dosages) were tested on eggs
and second stage larvae using three bio-
assay techniques namely water screen,
soil screen and soil with indicator plants.
The degree of control effected by each
chemical was estimated by counting the
nematodes which penetrated the roots of
Lycopersicum esculentum var. VC2-1 and
the number of galls formed therein.


Vol. 11







Jan. & June, 1975


. Hostathion, a contact insecticide-miti-
cide demonstrated a consistently efficient
control of M. incognita eggs and second
stage larvae at 10, 100 ppm and recom-
mended dosage in seven of the eight series
of experiments conducted. Percentage
control ranged from 66.73 to 99.97%. In
all concentrations, the nematicidal activi-
ty of the four chemicals were more dras-
tic on second stage larvae. At the highest
(100 ppm) dosage, all were lethal on both
the eggs and second stage larvae.
It is evident that the four nematicides
used here are effective as pre-plant con-
trol of M. incognita as long as the recom-
mended dosages are used.
The effect of nitrogen on lesion deve-
lopment of bacterial blight. R. Sapin,
O. A. Garcia, S. D. Merca and H. E.
Kauffman.
The effect of two nitrogen levels (high
4% and low 2%) on bacterial blight lesion
development was assessed on leaves of
IR8 (susceptible to both test isolates) and
IR20 (resistant to one and moderately
susceptible to the other test isolate)
under controlled conditions in the phy-
totron with day/night temperature of
32/24 C. High nitrogen enhanced lesion
development when inoculated with both
isolates in the resistant variety IR26, but
had no effect in the susceptible variety
IR8. This indicates that nitrogen content
in the leaf may differentially affect tissue
susceptibility to the causal organism
(X. oryzae).
Pathologic behavior of Sclerospora
philippinensis Weston on seven inbreds
and three varieties of corn under con-
trolled environmental conditions. Jas-
want Singh and 0. R. Exconde.
Sclerospora philippinensis Weston iso-
lated from UPCA var. 3 at College, La-
guna was studied under controlled envi-
ronmental conditions using "KOITO-


TRON" growth chambers to determine
whether temperature-relative humidity
exert influence on development of symp-
tom pattern as well as on its morphology
and physiology.
On seven inbreds and three varieties
of corn, four distinct symptom patterns
were distinguished viz., alternate chloro-
tic stripe, mosaic type, broken stripe, and
necrotic flecks.
Conidial size appeared to be directly
correlated with early systemic infection
in inbred PF 89-3-1.
High temperature-relative humidity
seemed to favor conidial production and
systemic infection at the early stage of
plant growth; lower temperature-relative
humidity completely inhibited conidial
formation. None of the inbreds and
varieties inoculated with about 45,000
conidia/ml was found resistant to the
causal organism under the conditions of
the test.
Plant parasitic nematodes associated
with coconut roots in the Philippines. -
R. B. Valdez, D. T Teru and A. S. dela
Cruz.
A survey for plant parasitic nema-
todes associated with the roots of 15-50
years old coconut trees was conducted.
It involved 770 samples collected from
77 towns and cities in 15 provinces,
namely, Laguna, Quezon, Cavite, Ba-
tangas, Camarines Sur, Albay, Cagayan,
Pangasinan, Leyte, Samar, Masbate, Misa-
mis Oriental, Davao, Lanao del Sur and
Zamboanga del Sur. Seventeen genera
were identified, namely, Rotylenchulus,
Helicotylenchus, Xiphinema, Cricone-
moides, Meloidogyne, Hoplolaimus, Pra-
tylenchus, Tylenchorhynchus, Ditylen-
chus, Longidorus, Trichodorus, Hemicy-
cliophora, Rotylenchus, Paratylenchus,
Tylenchulus and Aphelenchoides.
Of these, Rotylenchulus, Helicotylen-


ABSTRACTS








Philippine Phytopathology


chus and Xiphinema were the most abun-
dant with population counts that ranged
up to 5,357; 3,356 and 475 per 300 ml,
respectively. They were also widely dis-
tributed having been found in 93-100 per
cent of the sampling localities. Cicone-
moides, Meloidogyne, Pratylenchus, Ho-
plolaimus and Tylenchorhynchus, which
occurred in appreciable numbers, were
found in 80-88 per cent of the samples.


The other genera occurred from low to
very low counts and appeared to be of
less economic significance. Of the three
known vectors of soil-borne plant viruses,
namely, Xiphinema, Longidorus, and Tri-
chodorus, the first with four identified
species was the commonest. Xiphinema
was also widely distributed and was not
observed only in the samples from Davao.


Vol. 11








Phil. Phytopathol. 11:11-20
Jan. & June, 1975


EXPERIMENTAL EPIDEMIOLOGY OF RICE TUNGRO DISEASE

I. Effect of Some Factors of Vector (Nephotettix
virescens) on Disease Incidence

K. C. LING
Plant Pathologist
The International Rice Research Institute, Los Bafios, Laguna

The writer wishes to thank Dr. Kwanchai A. Gomez for statistical assistance and
her helpful suggestions on experimental design, Dr. Joyce C. Torio for editing the
manuscript, and Messrs. M. P. Carbonell and M. E. Mundin for technical assistance.


ABSTRACT

The effects of several factors related to the insect vector Nephotettix virescens
on the incidence of rice tungro disease in terms of percentage of infected seedlings of
the rice variety Taichung Native 1 (TN1) were studied by a cage method in a greenhouse.
Unless specified otherwise, virus-free adult or nymphal insects were introduced into the
cages and confined for 7 days in cages containing 300 test seedlings in 12 pots plus four
pots of diseased plants as the virus source. At the end of the tests, the insects were
counted and released while the pots were transferred to the greenhouse for the seedlings
to develop symptoms of tungro.
The percentage of infected seedling (Y) increased with an increasing number of
adult insects (X) introduced into the cage by the following relationship: Y = 100 100/
(1 + aX + bX2), where a and b varied according to the mortality of the insects during the
7-day period.
When 180 adult insects were introduced into each cage and confined for a number
of days (X), the percentage of infected seedlings'(Y) increased as the duration of con-
finement lengthened by Y = 100(1 e-0.1368X), where e = 2.718.
Based on the percentage of infected seedlings, the adult insects appeared to be
about three times more efficient in spreading the disease than the nymphs.
Watering the seedlings in the cages during the testing period caused the insects to
move, increasing the percentage of infected seedlings. The ratio of percentage of infected
seedlings with watering to the percentage of infected seedlings with no watering was
1.20:1 for adults and 2.65:1 for nymphs.


"When we think of a disease we should
think of the disease triangle: host, patho-
gen, environment" noted van der Plank
(1963). The disease triangle, a two-dimen-
sional figure, remains theoretically un-
changed even with plant virus diseases
that are transmitted by insect vectors
because the insect vectors are considered
a part of the environment.
"Epidemiology is the science of disease
in populations" (van der Plank, 1963).


The concept of a disease triangle could
be modified when a disease is considered
in terms of populations. If a plant virus
can be transmitted only by insect vectors,
the incidence of that virus disease in a
population of host plant would depend
to a large extent upon the population
density and activity of the insect vectors,
the other factors the host plant, the
virus, and the environment being con-
stant. The role of the insect vector in








Philippine Phytopathology


the epidemiology of the disease is very
important. Additionally, the insect vector
interacts with other factors environ-
ment, the host plant, and the virus. There-
fore, in an epidemiological study of a
plant virus disease transmitted by an
insect, the insect vector could be con-
sidered an independent actor rather than
grouped as part of the environment.
Thus, the disease triangle could be
changed into a disease triangular pyramid
to separate the insect vector from the
environment.
With the three-dimensional figure of
the pyramid, the incidence of an insect-
transmitted plant virus disease in popula-
tions is shown as a function of the host,
the virus, the insect vector, and the envi-
ronment groups of factors that affect
the outbreak and spread of the disease.
Studies on the experimental epidemiology
of a disease are designed to investigate
the effect of groups of factors, individual
or combined, on the incidence of the
disease in populations.
Studying the effect of groups of'fac-
tors on the incidence of the disease in
one experiment is complicated by the
interaction of the various factors which
comprise a single group. For example,
the insect vector group includes the in-
sect's population density or number of the
insect vectors per unit area, activity of
the insect vector, developmental stage of
the insect vector, and others. The location
of the virus source, the quantity, the
quality, and others are factors of the
virus group. Included among the factors
of the host plant group are susceptibility
of the host plant to the virus, its suscepti-
bility to the insect vector, and its growth
stages. The environment group includes
both biological (parasites, predators, alter-
native hosts, etc.) and nonbiological
(chemicals, temperature, etc.) factors.
When these factors are defined and se-
parated, and if a method of study is


available, their individual or combined
effects on the incidence of the disease in
terms of percentage of infected seedlings
in a plant population can be determined
experimentally.
The above concept was applied in this
study of epidemiology of rice tungro
disease which used the cage method to
investigate factors related to the insect
vector, Nephotettix virescens (Distant),
on the incidence of this disease.

MATERIALS AND METHODS

The cage method designed for experi-
mental epidemiological studies of the
tungro disease of rice (Ling, 1974) was
used to study the effect of four factors
associated with the insect vector on the
percentage of infected seedling number
of insects, number of rice seedlings,
Length of confinement of insects with the
rice plants, and watering of the plants.
While one factor was being tested,-the
three other variables, unless specified
otherwise, were arbitrarily standardized
as follows: 180 adult insects as the initial
number of the insects per cage, 7 days as
the duration of confinement of the in-
sects in the cage, 300 test seedlings in 12
pots and four pots of diseased plants as
the virus source in each cage, and the pots
of diseased plants placed at the second
position of the four side rows in the cage
(Fig. 1). The cages were arranged in rows
under a glass roof. Other environmental
factors were not controlled.
A colony of the insect vector, the rice
green leafhopper (N. virescens) was reared
on Taichung Native 1 (TN1) rice plants
in cages under Los Bafios, Philippines
conditions. Insects from the same colony
were used in different treatments of a
trial. The insects differed in age by no
more than 48 hours for each trial; the
eggs of each colony were collected within
2 days. The insects were virus-free when


Vol. 11








N. virescens and Tungro Incidence


Infected
100 v-


seedlings (%)


200 300 4
Insects (no./coge)


Fig. 1. Effect of number of Nephotettix virescens adults on the percentage of tungro-
infected seedlings when different numbers of the insects were confined in cages
containing 300 Taichung Native 1 test rice seedlings in 12 pots (open circles)
and four pots of diseased plants (solid dots) for 7 days.


they were introduced into the center of
each cage. At the end of the test period,
the insects were counted to ascertain how
may had died.
Unless specified otherwise, 11 to 13
days after the TN1 seeds were soaked, the
seedlings were transplanted into pots in
five rows at 25 seedlings per pot. After
the insects were released at the end of
the test period, the seedlings were trans-
ferred to the greenhouse for symptom
development. Both the infected and non-
ihfected seedlings were counted; those
seedlings which died before the reading
either because of the insect's direct da-
mage or due to unknown causes were
excluded from the count. The percentage
of infected seedlings was used to indicate
the incidence of the tungro disease.
The diseased TN1 plants used as the


virus source were inoculated by viruli-
ferous insects in the greenhouse.
For determining the effect of number
of insects encaged with rice seedlings on
the percentage of infected seedlings, 0, 15,
30, 60, 120, 180, 240, 300, 450, and 600
adult insects were introduced into indi-
vidual cages. The test included two dif-
ferent arrangements of the virus source
in the cages, replicated four times. In
April and May 1973, the pots containing
diseased plants were arranged diagonally
in the cages; the total number of seedlings
tested was 10,469. In August and Septem-
ber 1973, the pots of diseased plants
were placed at the second position one
pot in each of the four side rows of the
cage; a total of 10,076 seedlings was
tested.
When the effect of a constant number


Jan. & June, 1975








Philippine Phytopathology


of insects per seedling on the disease in-
cidence was studied, each cage contained
16 pots 12 pots with test seedlings
and four pots with diseased plants as
virus sources. One cage contained 1 seed-
ling per pot; another, 2 per pot; and
others, 5, 10, or 25 seedlings per pot.
On the basis of 0.6 insect/seedling for all
treatments, 8, 15, 36, 72, or 180 adult
insects were introduced into the cages
according to the number of seedlings per
pot in the cage. Since the total number
of test seedlings in a cage with only one
or two seedlings per pot was not large
enough to permit the calculation of per-
centage of infection, two extra cages for
these two treatments were added in each
trial. The test was repeated four times in
June to August 1973, using a total of
2,117 tested seedlings.
To examine the effect of duration of
confinement of the insects with the rice
seedlings on the incidence of the disease,
adult insects were introduced into each
cage and confined for 1, 2, 4, 7, 11, 16,
and 22 days. The test was repeated four
times, using a total of 7,467 test seedlings,
in June and July 1973.
To investigate the effect of growth
stage of the insect vector on the inci-
dence of the disease, 56 comparisons were
made on various dates between nymphs
(third instar or older) and adults, using a
total of 10,837 and 10,725 test seedlings
for nymphs and adults, respectively.
Watering the rice seedlings in the cage
caused the insects to move, resulting in a
higher percentage of seedling infection.
To determine the effect of watering seed-
lings during the test period, nine pots
with test seedlings were placed in a con-
tainer in each cage. The container held
enough water to flood the pots so that
no watering was required for 7 days. A
similar arrangement was used when the
seedlings were watered, except the pots


were not flooded with water but were
watered once or twice daily. Either
nymphs or adults were used in this study,
making two variables and a total of four
treatments. The six cages for each treat-
ment were replicated four times, involving
a total of 14,484 test seedlings.


RESULTS

Effect of number of insects
on disease incidence

Constant number of seedlings per cage.
- When each cage contained 300 test
seedlings and the virus source was ar-
ranged diagonally in the cage, the infected
seedlings increased from 0 to 91.5 percent
with an increasing number of insects from
0 to 600 per cage or from 0 to 2 per test
seedling (Fig. 1). Fifty-two percent of all
the test seedlings became infected; 37
percent of the insects died during the test
period.
When each cage contained 300 test
seedlings and the virus source was placed
at the side rows, the infected seedlings
also increased from 0 to 88.9 percent as
the number of insects increased from
0 to 600 per cage (Fig. 1). Forty-two
percent of the total test seedlings became
infected, and 62 percent of the insects
died during the test period.
The percentage of the infected seed-
lings (Y) depended on the number of
insects in the cage (X). The following
equation expressed the relationship of
these two factors:
Y = 100 100(1 + aX + bX2).
When the virus source was arranged dia-
gonally in the cage and the insect mortali-
ty during the test period was 37 percent,
a = 0.008 and b = 0.000022; when the
virus source was placed at side rows and
the insect mortality was 62 percent,
a =0.004 and b = 0.000013.


Vol. 11







N. virescens and Tungro-Incidence


The relation between the percentage
of infected seedlings and number of
insects in the cage was not a straight line
because the percent of infected seedlings
per insect did not increase at a constant
rate. The rates varied from 0.03 to 0.74
percent infected seedling/insect. The high-
est rate of infection occurred when the
number of insects was increased from
0 to 15. In general, the rate of increase
decreased gradually as the number of
insects in the cage increased.
Variable number of seedlings in a cage.
- When the number of seedlings and
the number of insects in a cage varied
but the number of insects per seedling
remained constant at the beginning of the
experiment, the number of seedlings in-
fected ranged from 4 to 162 in a cage
among treatments in all trials. The num-
ber of infected seedlings increased as the
number of insects was increased from 8
to 180 per cage with a constant ratio of
0.6 insect/seedling. However, the infected
seedlings increased from 36 percent when
8 insects and 12 test seedlings were in the
cage to 62 percent when 72 insects and
120 test seedlings were in the cage. The
percentage of infected seedlings decreased
slightly when the number of insects was


further increased to 180, with 300 test
seedlings in the cage (Table 1).
A similar trend was seen when the per-
centage of the infected seedlings were ma-
thematically adjusted according to the
number of insects that remained in the
cage at the end of test period (Table 1).

Effect of duration of insects' confine-
ment with rice seedlings on disease in-
cidence.

The infected seedlings increased from
8.8 to 93.6 percent as the duration of the
insects' confinement in the cage lengthen-
ed from 1 to 22 days (Fig. 2). The per-
centage of infected seedlings (Y) depend-
ed on the duration in days (X) of the
insects' confinement in cage as expressed
in the following equation:

Y = 100(1 e-0.1368X),

where e = 2.718. The increase in per-
centage of infected seedlings was not
directly proportional to the number of
days when confinement was prolonged;
the rate varied from 1.0 to 12.8 and was
highest between 0 to 1 day. Nevertheless,
the rate of increase decreased with pro-
longed duration of insects' confinement.


Table 1. A comparison of percentages of tungro-infected seedlings of rice variety
Taichung Native 1 in cages for 7 days with different densities of Nephotettix
virescens adults and using various numbers of test seedlings.

Insects Seedlings' Infected seedlings2 (%)

no./ no./ no./ no./ Observed Adjusted
seedling cage pot cage mean mean3

0.6 8 1 12 35.5 a 41.1 a
0.6 15 2 24 44.2 ab 45.0 a
0.6 36 5 60 49.5 ab 47.7 ab
0.6 72 10 120 66.2 c 63.3 b
0.6 180 25 300 57.5 bc 55.8 ab
1 Excluding four pots of diseased plants as a source of the virus in each cage.
2 In each column, means followed by a common letter are not significantly
different at the 5% level.
3 Adjusted by the mortality of the insects at the end of testing period.


Jan. & June, 1975







Philippine Phytopathology


Infected seedlings (%)
100


80-


12
Duration (days)


Fig. 2. Effect of duration of confinement of 180 Nephotettix virescens adults in cages
containing 300 test seedlings in 12 pots and four pots of diseased plants on the
percentage of tungro-infected Taichung Native 1 rice seedlings.


The seedlings infected when confined
with the insects for 1 day illustrated
that the originally virus-free insects com-
pleted the transmission of the tungro
virus from diseased plants to test seed-
lings in the cage within 24 hours.

Effect of developmental stages
of the insect on disease incidence
When the insects at nymphal stage
were confined in the cage, an average of
20.2 percent of the test seedlings became
infected, with a range of 3 to 92 per-
cent of infected seedlings per cage. When
adults were confined in the cage, 62.4
percent of the seedlings became infected,
with a range of 16 to 95 percent of in-
fected seedlings per cage. The difference
in the percentage of seedlings infected by
the nymphs and the adults was statistical-


ly significant. The wide range in the per-
centage of infected seedlings per cage can
be attributed to the difference in the
number of insects per cage.

Effect of watering on disease incidence
Watering appeared to increase the per-
centage of infected seedlings; 46.2 per-
cent of the seedlings were infected when
the plants were watered in the cage,
while only 31.9 percent were infected
when the plants were not watered during
the time the insects were confined with
the rice seedlings. While the ratio of per-
centage of infected seedlings occurring in
the watered and nonwatered treatments
was 1.45:1, the ratio changed depending
upon the stage of development of the
insect. For the adult insects, the ratio of
percentage of infected seedlings between


Vol. 11







N. virescens and Tungro Incidence


watering and no watering was 1.20:1,
while for the nymphs, it was 2.65:1. The
increase in percentage of infected seed-
lings due to watering was greater for the
nymphs than that for the adults (Table 2).


of the leafhopper population. Similarly,
in Malaysia, Lim, Ting, and Heong (1974)
found that a high leafhopper population
was closely associated with a high inci-
dence of tungro; additionally, when the


Table 2. A comparison of percentages of tungro-infected seedlings between test seedlings
in a cage watered daily and not watered.

Total seedlings
Nephotettix infected/tested Mean of infected
virescens Treatment (no.) seedlings1 (%)

Adults Watering 2348/3761 63.2
No watering 1801/3499 52.8

Nymphs Watering 1017/3700 29.2
No watering 372/3524 11.0

Average of 24 trials each. Paired t test = 2.82** and 5.36** for adults and nymphs,
respectively.


DISCUSSION
Population is a function of three
variables; births, deaths, and migration.
There are four ways in which the number
of insects in a rice field can be changed:
1) new insects may be born in the field;
2) insects in the field may die; 3) insects
may move in from the outside; and 4) in-
habitant insects may move out. The
changes can occur at anytime, and thus,
the number of insects in a field is con-
tinually changing. When insects are con-
fined in a cage, the metal screen prevents
migration. However, the births, the deaths,
and the growth of the insects cannot be
controlled in studies on the effect of
number of N. virescens or of the growth
stages of the insect on the incidence of
tungro disease by the cage method.
The reports in the literature on the
effect of leafhopper vector population on
the incidence of tungro disease differ.
Lippold et al. (1970) suggested that the
severity of tungro in the 1968 aman
(monsoon season, June to December) in
Bangladesh might have been influenced
by climatic conditions favoring a buildup


population was low, the tungro infection
was either absent or insignificant. On the
other hand, in Mindanao, Philippines, a
field of Taichung Native 1, a rice variety
highly susceptible to the tungro disease,
had no tungro disease although there was
a high population of N. virescens (Ling,
1972). Mukhopadhyay and Chawdhury
(1973) found that the highest leafhopper
population in West Bengal, India did not
correspond to the highest incidence of
the tungro disease and in aus (autumn
rice, June to September) season, and that
the spread of tungro could be extensive
even with a very low leafhopper popula-
tion. These reported findings do not con-
tradict, and can be explained by the con-
cept of the disease triangular pyramid of
rice tungro although population of the
leafhopper vector is a major factor affect-
ing the incidence of the disease, it is not
the only factor determining the incidence
of the disease in a given area.
The number of insects in a cage de-
termined the percentage of tungro-in-
fected seedlings in the cage when the
number of seedlings per cage and other
factors remained unchanged (Fig. 1). The


Jan. & June, 1975







Philippine Phytopathology


larger the number of leafhoppers per unit
area, the higher was the percentage of
infected seedlings, although the increase
in the rate of the percentage of infected
seedlings per insect was not constant
(Fig. 1). Hence, the population density
of the leafhopper is a major factor af-
fecting the incidence oftungro. However,
when other factors vary, a high popula-
tion density of the leafhopper may not
necessarily result in a high incidence of
the tungro disease since these other fac-
tors, i.e., availability of virus source, sus-
ceptibility of rice plant, and environ-
mental conditions, affect the incidence of
the tungro.

On the other hand, an extremely low
population of the leafhopper in an area
is less likely to cause an outbreak or a
high incidence of the tungro disease in
the area even when the other factors favor
the outbreak of the disease. Hence, an
extremely low population of the leaf-
hopper can limit the outbreak of the
disease. Consequently, an extremely low
population of the leafhopper vector in an
area can serve as a criterion for "Negativ-
prognose" (negative prediction) (Schr8d-
ter and Ullrich, 1966) for an outbreak of
the tungro disease in that area although
the negative forecasts are generally based
on arithmetical weighing of weather con-
ditions for as long as disease appearance
is considered highly unlikely (Bourke,
1970).

The percentage oftungro-infected seed-
lings varied even though the number of,
insects per seedling was constant (Table
1), perhaps because the density of the
insect population may not directly relate
to the number of insects per plant unless
the number of plants per unit area is
constant. When the number of plants in
an area varies, the distance between two
plants changes if the plants are evenly
distributed in that area. Changing the


distance between two plants may affect
the spread of the tungro disease by the
insects. Moreover, when the population
density of the insect is high, the chance
that several insects will be on one leaf is
greater. When several insects are feeding
on a leaf, and one insect moves away or
another leafhopper moves to the leaf, the
other insects on the leaf are disturbed by
the insect and tend to move from the leaf,
resulting in a difference in spread of
tungro.
However, when the number of insects
per seedling remains unchanged, the per-
centage of infected seedlings can increase
to a certain extent with an increase in the
density of the leafhopper population. If
the number of insects per seedling remains
unchanged, then an increase in the popu-
lation density of the leafhopper is con-
comitant with an increase in the number
of seedlings in a unit area. Therefore, the
actual infected seedlings may be high in
an area when the population density of
the leafhopper is high but the percentage
of infected seedlings may not be high
because of the high number of seedlings
in that particular area. Hence, the inci-
dence of tungro disease in terms of per-
centage of infected seedlings depends not
only on the population density of the
leafhopper but also on the number of
plants per unit area.
The percentage oftungro-infected seed-
lings increased when the duration of con-
finement of the leafhopper with the rice
seedlings in the cage was prolonged even
though the number of the insects re-
mained unchanged (Fig. 2). Hence, the
length of time leafhopper vectors remain
in a rice field affects the incidence of
tungro disease when other factors are
constant. However, this factor of duration
cannot be readily separated from the
insect population factor because shorten-
ing the duration of the insects in a
field reduces the insect population in the


Vol. 11









N. virescens and Tungro Incidence


field. When insecticide is applied to a
field to kill the insects or to cause them
to leave the field, it not only reduces the
insect population but also shortens the
duration of the time the insects are in
in the field.
However, time is a factor determining
the amount of plant disease (van der
Plank, 1975) and epidemics change in
time (Kranz, 1974). Obviously, the time
interval is important to the duration of
the insects in a field; delaying the control
of the insect vector in a field could im-
pair the effectiveness of the treatment.
Nevertheless, a high population of the
insect vector for a short duration in a
field cannot increase the incidence of the
tungro disease over that effected by the
same high population of the insect vectors
with a longer duration in the field, unless
the short duration is sufficient to allow
the insects to effect 100 percent plant
infection.
Mortality of the leafhopper contributes
to the experimental errors in the results
obtained from studying the effect of the
insects on the incidence of tungro disease
by the cage method. Since the death of
insects or their life span cannot be con-
trolled under natural conditions and varied
among trials, the percentage of infected
seedlings varies accordingly. The main
reason for the difference in percentage of
infected seedlings between the two treat-
ments of arranging the pots of diseased
plants diagonally or at the side rows of
the cage (Fig. 1) 52 versus 42 percent -
could be the mortality of the insects
during the test period. The tests were not
made by using the same colony of the
insect on the same date; 37 percent of
the insects died at the end of test period
when the pots of diseased plants were
arranged diagonally and 62 percent died
when the diseased plants were placed on
each of the four side rows of the cage.


The percentage of infected seedlings
could be mathematically adjusted by the
number of insects remaining in the cage
at the end of the test period (Table 1).
The adjusted means may not be theore-
tically reliable because the mean does not
include the time of death of the insects,
which could have affected the incidence
of the disease because the insect that
died earlier would have had less chance
to spread the disease in a given time.
When other factors remained un-
changed, the adult insects appeared to
be about three times more efficient in
spreading the disease than the nymphs.
This difference may be due to: 1) nymphs
cannot fly; so, the distance they travel to
reach a diseased plant to acquire the
virus or to reach a healthy plant to in-
troduce the virus is shorter than that of
an adult (the spread area); and 2) the
nymph should take longer to cover a
given distance within a specific reachable
range than the adult. Hence, nymphs will
probably move between two plants less
frequently than the adults in a given
time. Furthermore, the infectivity of the
insect decreases with increasing time after
acquisition feeding (Ling, 1966). Hence,
if the insect takes longer to reach a
healthy plant after acquisition feeding,
the infectivity will be lower when the
insect reaches the healthy plant. However,
when the distance between the insect and
the rice plant, between a diseased and a
healthy plant, and between two healthy
plants is shorter than the distance of one
jump of the nymph, the difference in the
spread of tungro between the nymph and
the adult would be small.
Another limitation of the nymph in
spreading the disease is molting. During
molting the insects are practically in-
active, and therefore cannot spread the
disease. In addition, molting terminates
the infectivity of the insects (Ling, 1966).


Jan. & June, 1975









Philippine Phytopathology


The insect becomes infective again only
after another acquisition feeding on a
diseased plant. On the other hand, the
adult leafhopper cannot spread the disease
during mating and the female cannot
spread the disease while laying eggs.

Watering the rice seedlings in the cage
while the insects were confined with the
plants increased the percentage of tungro-
infected seedlings (Table 2). Hence, even


if all the above mentioned factors of the
leafhopper Were constant, the incidence
of tungro disease still could be increased
by any agent that induces the insects to
move more frequently from plant to
plant. It is difficult to standardize and to
maintain the rate at which the insects are
induced to move because it involves both
the response of the leafhoppers to the
agent and the behavior of the insects at
the moment they receive the stimulus.


LITERATURE CITED

BOURKE, P. M. A. 1970. Use of weather information in the prediction of plant disease
epiphytotics. Annu. Rev. Phytopathol. 8:345-370.
KRANZ, J. 1974. The role and scope of mathematical analysis and modeling in epi-
demiology, p. 7-54. In J. Kranz (ed.) Epidemics of plant diseases. Springer-Verlag,
New York Heidelberg Berlin.
LIM, G. S., W. P. TING, and K. L. HOENG. 1974. Epidemiological studies of tungro
virus in Malaysia. Paper presented at the International Rice Research Conference,
Los Bafips, Philippines, April 22-26. 13+8 p. (mimeo.)
LING, K. C. 1966. Nonpersistence of the tungro virus of rice in its leafhopper vector,
Nephotettix impicticeps. Phytopathology 56: 1252-1256.
LING, K. C. 1972. Rice virus diseases. International Rice Research Institute, Los Bailos,
Philippines. 142 p.
LING, K. C. 1974. A cage method for studying experimental epidemiology of rice tungro
disease. Phil. Phytopathol. 10: 31-41.
LIPPOLD, P., G. E. GALVEZ-E., M. S. A. MIAH, and M. S. ALAM. 1970. Rice tungro
virus in native population of Nephotettix impicticeps in East Pakistan. Int. Rice
Comm. Newslett. 19(1):18-23.
MUKHOPADHYAY, S. and A. K. CHAWDHURY. 1973. Some epidemiological aspects of
tungro virus disease of rice in West Bengal. Int. Rice Comm. Newslett. 22(4):44-57.
SCHROPDTER, H. and J. ULLRICH. 1966. Weitere Untersunchungen zur Biometeorologie
und Epidemiologie von Phytophthora infestans (Mont.) de By. Ein neues Kon-
zept zur L5sung des Problems der epidemiologischen Prognose. Phytopath. Z.
56:265-278.
VAN DER PLANK, J. E. 1963. Plant diseases: epidemics and control. Academic Press,
New York and London. 349 p.
VAN DER PLANK, J. E. 1975. Principles of plant infection. Academic Press, New York,
San Francisco, London. 216 p.


Vol. 11








Phil. Phytopathol. 11:21-31
Jan. & June, 1975



EXPERIMENTAL EPIDEMIOLOGY OF RICE TUNGRO DISEASE

II. Effect of Virus Source on Disease Incidence

K. C. LING
Plant Pathologist
The International Rice Research Institute, Los Bafios, Laguna

The author is grateful to Drs. Kwanchai A. Gomez and Douglas L. Neeley for their
statistical assistance and to Messrs. M. P. Carbonell and M. E. Mundin for their technical
assistance.

ABSTRACT

The effect of virus source on the incidence of rice tungro disease in terms of
percentage of infected seedlings of the rice variety Taichung Native 1 (TN1) was
determined by the previously developed cage method. When the amount of virus source
in a cage varied, the percentage of infected seedlings (Y) increased as the percentage of
pots of diseased plants (X) increased by Y = a + bX for the tested amount of virus source
and by Y = aX + bVX when zero infection was included for no diseased plants and 100
percent infection for all diseased plants in the cage. The a and b values varied according
to the growth stage of the insect, Nephotettix virescens. When amounts of virus sources
were equal, the arrangement of diseased plants in a cage affected somewhat the disease
incidence. A significantly higher percentages of seedlings adjacent to the virus source
were infected. Cages containing virus sources of diseased plants of TN1 or IR22 had
significantly higher percentages of infected seedlings than had cages with diseased plants
of C4-63G or IR20. Hence, diseased plants of different varieties were not identical in
.quality of the virus source.


Matthews (1970) wrote, "It is obvious
that there will be no virus problem if the
crop is free of virus when planted and
when there is no source of infection in
the field or none near enough to it to
allow spread into the crop." For epi-
demiological studies of an insect-trans-
mitted virus, the concept of disease in-
cidence as a function of the virus has been
illustrated by a three dimensional triangu-
lar pyramid consisting of host, virus, en-
vironment, and insect vector on the basis
of disease in populations (Ling, 1975).
When noninfected plants in a field are
emphasized, "virus" automatically be-
comes the virus source for the disease
incidence. This stresses that the source of
virus plays an important role in disease
incidence in the field when other factors
are equal.


The source of an insect-transmitted
virus can only be the diseased plant or
the insect vector, or both (some viruses
multiply in both plant and vector). But
in rice tungro disease, the virus source
can only be the diseased plants since
the virus not only does not multiply
in the vector but also does not persist in
the vector (Ling, 1966). The virus is
transmitted to the healthy plants, how-
ever, by the insect vector. Therefore the
primary virus source for a rice seedbed
could be located outside of the seedbed
and incoming viruliferous insects carry
the virus into the seedbed. This is sup-
ported by the following factors: 1) the
rice seedbed is generally very well pre-
pared, which means there is commonly
neither regenerated growth of rice stubble
nor weeds; 2) there is lack of evidence to









Philippine Phytopathology


indicate that tungro virus is transmitted
through seeds or by other means (Ling,
1972); and 3) rice seeds are not the
carriers of viruliferous insects. Once the
seedlings are infected by the incoming
viruliferous insects, they would act as virus
sources for the noninfected seedlings in
the seedbed.
The primary virus source for a rice
field could be located either outside or
inside the field. The incoming viruliferous
insects carry the virus from outside into
the field. The diseased plants inside the
field could be from the previous crop or
could be infected in the seedbed and
transplanted into the field.
The effect of a virus source located
outside a field on the field incidence of
tdngro disease can be likened to the ex-
perimentally demonstrated effect ofviru-
liferous insects on the tungro incidence
in terms of percentage of infected seed-
lings in a cage (Ling, 1974). The objec-
tive of this study was to determine the
effect of diseased rice plants inside a field,
acting as virus sources, on the incidence
of tungro disease.
The virus source, as it affects the in-
cidence of tungro disease, can be broken
down into several components. The quan-
tity of virus source refers to the diseased
plants, either in number per unit area or
in percentage of total plants per field.
The quantity of virus source in a rice
field may affect the plant disease inci-
dence in the field because it is funda-
mentally equivalent to the relation be-
tween the amount of inoculum and the
amount of disease produced (van der
Plank, 1975). The location of virus source
is another component. If a diseased plant
is located beyond the distance that insect
vectors in a rice field can reach, it ob-
viously cannot contribute directly to the
disease incidence in the field. A third
component is quality of virus source,


which is the percentage of insects that
become infective after feeding on diseased
plants of different kinds or the percentage
of seedling infection as a result of using
the diseased plants. Not all diseased plants
are equally good sources of tungro virus
for the insect vector (IRRI, 1973, 1974;
Narayanasamy, 1973). This paper reports.
the results of studies of the effects of
these components of virus source on the
incidence of tungro disease, as determined
by the cage method.
MATERIALS AND METHODS

The cage method developed by Ling
(1974) for studying experimental epi-
demiology of rice tungro disease was
used. The cage contains all essentials for
tungro disease to spread to the test seed-
lings insect vectors, test seedlings, and
diseased plants that serve as virus sources.
Each factor to be tested is varied among
the cages, but the other factors remain
constant in all cages. The percentage of
infection of the test seedlings is used to-
illustrate the effect of the factor on the
disease incidence. In this study, the vary-
ing factor was aspect of diseased rice
plants that act as virus sources.
The insect vector, the rice green leaf-
hopper Nephotettix virescens (Distant),
was reared on rice plants of the variety
Taichung Native 1 (TN1) in screen cages
at Los Bafios, Philippines. The insects
from eggs collected in single colonies
during 2-day periods were used for dif-
ferent treatments. The insects were free
of virus when introduced by releasing
them at the center of the cages at the
rate of 180 adults or nymphs per cage.
The insects were discarded at the end of
the 7-day test period.
The diseased plants used as virus
sources were prepared by inoculating
healthy seedlings of TN1 or other varieties
with viruliferous insects. The inoculated


Vol. 11








Virus Source and Tungro Incidence


seedlings were kept in the greenhouse and
used as virus sources when the disease
symptoms developed.
The 11- to 13-day-old TN1 test seed-
lings were transplanted in pots at 25
seedlings per pot. The pots of test seed-
lings, together with pots of diseased
plants, were placed in cages at 16 pots
per cage. The cages were kept under a
glass roof; other environmental condi-
tions were not controlled.
At the end of the 7-day test period,
the pots of test seedlings were transferred
to the greenhouse to allow the symptoms
to develop. Three to 4 weeks later, both
infected and noninfected seedlings of
each pot were counted. The number of
seedlings, therefore, did not include those
that died before symptoms developed.
The percentage of infected seedlings indi-
cated the incidence of the disease.
To determine if any seedling infection
occurred when the cage contained no
diseased plants as virus sources, four
cages were tested. Each contained only
180 adult insects and 16 pots of test
seedlings. The test was conducted four
times, involving a total of 6,041 test
seedlings in 256 pots.
To investigate the effect of the amount
of virus sources on the disease incidence,
one, two, four, and eight pots of diseased
plants of TN1 were placed in the cages
together with 15, 14, 12, and 8 pots of
test seedlings, respectively. In the cages
with one pot of diseased plants, the pot
was placed third in position of the second
row. In the cages with two pots of
diseased plants, one was placed in third
position, second row, and the other in
second position, third row. In the cages
with four pots of the diseased plants,
they were placed at the second position
of each of the four side rows. In the
cages with eight pots of the diseased
plants, the pots were alternately arranged


with pots of test seedlings. Each test was
conducted four times by using adults and
nymphs. The experiment involved a total
of 4,373 test seedlings for the adult
insects, and 3,940 test seedlings for the
nymphal insects.
To determine the effect of arrange-
ment of virus source in a cage on the
incidence of tungro disease, four pots of
diseased plants of TN1 and 12 pots of
test seedlings were arranged differently
in the cages. Four pots of diseased plants
can be arranged 1,820 possible ways in
the 16-pot cage. However, only 10 ar-
rangements were tested. The test was con-
ducted four times, involving 11,268 test
seedlings at rates of between 1,110 and
1,142 test seedlings per arrangement.
Two arrangements were particularly
studied to determine the relation between
seedling infection and distance between
test seedlings and virus source pots of
diseased plants placed at the second
position of each of the four side rows
and pots of diseased plants arranged dia-
gonally in the cage. For the former, 89
tests were made, involving 24,065 test
seedlings in 1,068 pots and 356 pots of
diseased plants. For the latter, 48 tests
were made, involving 12,653 test seed-
lings in 576 pots and 192 pots of diseased
plants. The average percentages of in-
fected seedlings of the two arrangements,
however, could not be compared because
the tests were made separately on dif-
ferent dates by using different colonies of
insects, different numbers of insects, and
different lengths of duration of insects'
confinement in the cages.
To examine the effect of quality of
virus source on the disease incidence,
one, two, four, and eight pots of diseased
plants of rice varieties C4-63G, IR20,
IR22, or TN1 were placed in the cages
together with pots of test seedlings to
make a total of 16 pots in each cage.


Jan. & June, 1975








Philippine Phytopathology


The test was conducted four times, in-
volving a total of 17,779- test seedlings
in 784 pots.


RESULTS

Without virus source. In the cages
that contained no diseased plants as virus
source, none of the 6,041 tested seedlings
showed symptoms of tungro disease, al-
though the seedling mortality was 5.6 per-
cent.
Quantity of virus source When the
number of rice plants in a unit area re-
mains constant, as more plants are .in-
fected with tungro virus, fewer non-
infected ones remained in the field. Si-
milarly, increasing the amount of virus
source in a cage resulted in more pots of
diseased plants and fewer pots of test
seedlings. Thus, the total number of test
seedlings per cage was reduced from 375
to 350 to 300 to 200 as diseased plants
increased from one to two to four to
eight pots. The mortality of the seedlings
tested ranged between 8 and 14 percent
when adult insects were used. It ranged
between 16 and 26 percent when nymphs
were used. At the end of the 7-day
period, the insect mortality ranged be-
tween 24 and 28 percent for the adult
insects and between 35 and 50 percent for
the nymphal insects. Between 40 and 49%
of the nymphs became adults at the end
of the test period.
As amounts of virus sources or pots of
diseased plants increased in cages, the per-
centage of infected seedlings also in-
creased, regardless of the insect growth
stage. The infected seedlings increased
from 42.8 to 84.7% as the virus source
was increased from one to eight pots of
diseased plants per cage when adult in-
sects were used, and from 22.7 to 53.4%
when nymphs were used (Fig. 1).
Within the range of the tested amount


of virus source from 6.25 to 50% (one
to eight pots of diseased plants in a 16-pot
cages) the relation between the per-
centage of infected seedlings (Y) and
amount of virus source in terms of per-
centage of pots of diseased plants (X)
was:
Y = a + bX.
For the adult insects a = 37.39 and
b = 0.9753. For the nymphal insects
a = 16.88 and b = 0.7346.
The rate of infected seedlings for adult
and nymphal insects did not increase
identically with the increase in amount
of virus sources. The infected seedlings
increased 0.96% for adults and 0.70% for
nymphs for each 1-percent increase of
number of pots of diseased plants when
the virus source in the cage ranged be-
tween 6.25 and 50%.
In theory and in practice, zero infec-
tion would occur if no virus source was
placed in the cage, and 100 percent in-
fection would occur if all plants in the
cage were infected. In Fig. 1, hypothetical
curves have been drawn through these
two points as well as the points experi-
mentally obtained. The relation between
percentage of infected seedlings and
amount of virus source could be described
as:
Y = aX + b /X-.
For the adult insects a = -0.673 and
b = 16.72. For the nymphal insects
a = 0.5263 and b = 4.737.
The rate of increase of infected seed-
lings per increase of amount of virus
source was no longer constant. The rate
gradually decreased as the amount of
virus source was increased regardless of
the insect growth stage.
Arrangement of virus source.- Depend-
ing on the arrangement of pots of diseased
plants, from 29 to 58% of the seedlings
in the cage became infected (Fig. 2).
The difference in percentage of infected


Vol. 11









Virus Source and Tungro Incidence


Infected
100 --


80 I


seedlings (%)


NYMPHS


0.5263X +4.737/X
(R= 0.992**)


40



20-


O'
0 25 50 75 100
Virus source (/ol of pots of diseased plants in 16-pot cage)


Fig. 1. Effect of amount of virus source on the percentage of tungro-infected seedlings
of Taichung Native 1 (180 Nephotettix virescens caged for 7 days; 4,373
seedlings for adults, 3,940 seedlings for nymphs).


Y= 16.88+0.7346X
(r= 0.990**)


Jan. & June, 1975







Philippine Phytopathology

2 41%






5 42%71
41 51 49 41
17 19 30 20

5 42%
28 34 45 43



24 51


Vol. 11


3 42%
37 50 51 32
46 46)
46 50
17 23 56 44


Average infected
seedlings (%)


= Virus source
(diseased plants)


Fig. 2. Percentage of tungro-infected seedlings of each pot in cages with various arrange-
ments of virus source (light source was from the top in the drawings; a total of
1,110 to 1,142 seedlings per arrangement).







Virus Source and Tungro Incidence


seedlings among the 10 arrangements was
not statistically significant, except for
that between the arrangements 1 and 10
(Fig. 2). The percentage of infected seed-
lings might be affected by differences in
source of light in relation to the location
of virus source in the cage, as well as in
relation to the numbers of pots of test
seedlings adjacent to the virus source.
For instance, the small difference in per-
centage of infected seedlings between
arrangements 1 and 4 (Fig. 2) might be
due to the direction of light source in
relation to the location of diseased plants;
the diseased plants placed in the bright
side of the cage might attract more in-
sects, resulting in a higher percentage of
infected seedlings because of a higher
number of viruliferous insects. This was
not the case, however, when the diseased
plants were placed together at one comer
of the cage (arrangements 2 and 5, Fig. 2)
or at the center of the cage (arrangement


3, Fig. 2). The difference between ar-
rangements 1 and 9 (Fig. 2) might be due
to the numbers of pots of test seedlings
adjacent to the virus source. The trend
was that as more pots of test seedlings
were placed adjacent to the pots of dis-
eased plants, the total number of in-
fected seedlings increased.

Neither of these arrangements could
restrict the distribution of infected seed-
lings to a certain portion of the cage.
Although the infected seedlings were scat-
tered in the cage regardless of arrange.
ment of the virus source, the percentage
of'infected seedlings was different in a
different portion of the cage. The trend
was that as the diseased plants were more
scattered, the distribution of infected
seedlings in the cage was more even. In
other words, the differences became small-
er in percentage of infected seedlings
among pots.


Location Seedlings
59 6catn Tested (no.) Infected (%)

O Corner 8042 52a

S Side 8008 56b

SCenter 8015 58b

Seedlings
Location Tested (no.) Infected(%)

53 A 2 2 1 48a

Q B 4255 57b

SC 6277 65c


S= Virus source (diseased plants)
Fig. 3. Percentage of tungro-infected seedlings of each pot at different distances from
the virus source in a cage (means followed by a common letter are not significant;
ly different at the 5% level).


Jan. & June, 1975








Philippine Phytopathology


Results from 89 tests revealed that the
surrounding amount of virus source in-
fluenced infection. The upper portion of
Fig. 3 shows that the pots at four comers
of the cage had significantly lower per-
centages of infected seedlings than those
either on the side or at the center of the
cage, although the difference was only
from 4 to 6% infected seedlings. The pots
at four comers were adjacent to only one
pot of diseased plants while others were
adjacent to two pots.
Results from 48 tests showed that
seedling infection was also influenced by
the distance between the test seedlings
and the virus source in the cage. The
bottom of Fig. 3 shows that the pots at
locations A, B, and C were at different
distances from the virus source. The dis-
tance between the test seedlings and the
diseased plants was A>B>C. On the con-
trary, the percentage of infected seedlings
was C>B>A; the differences were statis-
tically significant.
Quality of virus source.- The mortality
of test seedlings ranged between 6 and
12 percent among treatments of diseased
plants of four rice varieties used as the
virus source in the cage. The insect mor-
tality ranged between 40 and 42% among
the treatments at the end of the test
period.
The percentage of infected seedlings


varied among the treatments. When dis-
eased plants of TN1 or IR22 were used
as virus sources, the percentages of in-
fected seedlings were significantly higher
than when diseased plants of C4-63G or
IR20 were used (Table 1). However, no
significant differences in percentage of
infected seedlings were found either be-
tween TN1 and IR22 or between C4-63G
and IR20. Therefore, diseased TN1 and
IR22 plants were better sources of the
tungro virus than C4-63G and IR20.

DISCUSSION

"The rate at which a virus spreads
between plants varies widely according to
the type of virus, crop, environment, and
mode of transmission" (Thresh, 1971).
"Spread of plant viruses by insects de-
pends on the number of sources of the
virus, the number and activity of the
insects able to transmit it, the readiness
with which the insects become infective
and the length of time for which they
remain infective, and the susceptibility of
healthy plants to infection" (Broadbent,
1959). A virus source is obviously essential
for the virus to spread, particularly when
the insects remain infective for only a
short time. If the essential virus source is
lacking, the virus will not spread. The
blank test of this study demonstrated
that no seedlings with tungro symptoms


Table 1. Percentage of tungro-infected seedlings of the rice variety Taichung Native 1
when diseased plants of different varieties acted as virus sources in cages with
180 adult insects of Nephotettix virescens for 7 days.

Virus source TN1 seedlings Average infected
(variety) tested (no.) seedlings (%)

C4-63G 4502 32 a
IR20 4604 29 a
IR22 4326 63 b
TN1 4347 71 b

'Means followed by a common letter are not significantly different at the 5% level.


Vol. 11







Virus Source and Tungro Incidence


was observed in the cages when the in-
sects were virus-free without diseased
plants serving as a virus source. Con-
sequently, absence of diseased plants in
an area can serve as a criterion for "Nega-
tivprognose" (negative predication)
(Schridter and Ullrich, 1966) for the
outbreak of tungro disease in the area if
no incoming viruliferous insects carry the
virus from an outside source.
"When infected plants within a crop
are the only sources, the ultimate inci-
dence of disease may be directly pro-
portional to the initial incidence" (Broad-
bent, 1964). The disease plants placed in
the cage as the virus source can be con-
sidered as the initial incidence of the
cage. These results showed that the per-
centage of infected seedlings (Y), or
the ultimate incidence of the disease for
the given test period, increased at a
constant rate with the increasing amount
of virus source or the initial incidence
within the range of tested numbers of
pots of diseased plants in the cage (X,
when converted in percentage) because
Y = a + bX (Fig. 1). However, when two
extreme points were included 0 and 100
percent infection, respectively, for no
and for all diseased plants in the cage -
the rate was no longer constant because
Y = aX + b\/X. The a and b values varied
according to the insect growth stage used
for the experiment. The variation can be
explained by the fact that adult insects
are more efficient than nymphs in the
spreading of tungro disease (Ling, 1975b).
The location of virus source affects
the incidence of the disease. Due to the
limited size of the cage, the maximum
distance at which a diseased plant can
act as a direct virus source for a healthy
plant remains obscure. Obviously, the
distance is determined by how far a
viruliferous insect can travel. Available
information shows that Nephotettix cinc-
ticeps can travel a maximum distance of
41 m in a paddy field in 1 day (Miyashita


et al., 1964) and that the longest retention
period of tungro virus by N. virescens
is 6 days (Ling, 1972). Hence, a diseased
plant at a distance greater than 250 m
could no longer be a direct virus source
for the healthy plant. This distance is
suspected to be longer, however, because
of several reasons, such as: 1) a more
capable insect vector ofN. virescens may
travel a longer distance; 2) an insect may
passively travel further because of strong
wind; and 3) environmental conditions
may affect the retention period so that
the insect has a longer time to remain
infective for a further distance.
The frequency that an insect travels
between a diseased and a healthy plant
would affect the incidence of the disease
because the tungro virus does not persist
in the insect and the insect can only
become infective after acquisition feeding
on the diseased plant. In other words,
when a diseased plant is located at a more
convenient spot for the insect, the pro-
bability and frequency of the diseased
plant acting as a virus source would be-
come greater, which results in a greater
spreading of the disease. This could ex-
plain not only the effect of arrangement
of diseased plants in a cage on the per-
centage of infected seedlings (Fig. 2) but
also the effect of amount of virus source
on the disease incidence (Fig. 1). More
diseased plants acting as virus sources in
a unit area might increase the chance for
insects to feed on and acquire the virus
from them. Additionally as diseased plants
increase in a field, the average distance
between diseased and healthy plants would
shorten if the plant spacing remains
constant.
The distance between a virus source
and a healthy plant affects the infection
of the healthy plant. Figure 3 shows that
the seedlings in the proximity of the
virus source had higher percentages of
infection, probably because of the dis-
persal of the insects from the diseased


Jan. & June, 1975







Philippine Phytopathology


plants. Miyashita et al. (1964) pointed
out that the number of recaptured indi-
viduals of N. cincticeps decreased as dis-
tance increased from the point of release,
and that the relation was expressed by a
linear regression. Similarly, Ling and Car-
bonell (1975) reported that seedling-to-
seedling movements of tungro-viruliferous
N. virescens were higher between adjacent
seedlings. Therefore, a plant that is close
to a diseased plant has a greater chance
of being infected because it is more likely
to be visited and inoculated by the in-
sects dispersed from the diseased plant.
Diseased plants that act as virus sources
of tungro can be of any host species.
Although various investigators have ob-
tained contradictory data and most of the
results have not been thoroughly con-
firmed, the researchers (Wathanakul,
1964; Rivera et al., 1969; Ting, 1971;
Mishra et al., 1973; Ting and Ong, 1974)
have reported the following species of
plants infected with tungro virus and
similar diseases by inoculation with viru-
liferous insects:
Bothriochloa odorata
Brachiana reptans
Dactyloctenium aegyptium
Digitaria adscendens
Echinochloa colonum
E. crusgalli
Eleusine indica
Eragrostic tenella
Ischaemum rogosum
Leersia hexandra
Oryza barthii
Oryza fatua
0. officianalis


0. ridley
O. rufipogon
Paspalum distichum
P. scrobiculatum
Pennisetum typhoides
Setaria glauca
S. verticillata
Sorghum vulgare
Triticum aestivum

Furthermore, the natural infection of
Eleusine indica, Hemarthria compressa,
Polypogon monspeliensis, Sorghum hale-
pense, and Sporobolus tremulus in India
was reported by Mishra et al. (1973).
These lead to the conclusion that in
addition to diseased rice plants, infected
plants of other species cannot be neg-
lected as important virus sources in the
field. They may differ, however, in quali-
ty as virus sources.
Even diseased rice plants of different
varieties differed in quality as virus sources
in this study (Table 1). This phenomenon
is not unknown. Broadbent (1959) ex-
plained that "the acquisition of a virus
by a vector may depend on the species
of plant, its age, the distribution of virus
within it, or the insect's feeding site. The
reason why insects can acquire a virus
more readily from one species of plant
than from another are usually not known,
although differing virus concentration and
distribution, or possibly the presence of
virus inhibitors in some plants, may be
responsible." Furthermore, an insect vec-
tor's preference to a variety or a species
of plant may also be a reason for its
being a good source of virus.


LITERATURE CITED

BROADBENT, L. 1959. Insect vector behavior and spread of plant viruses in the field,
p. 539-547. In C. S. Holton (ed.), Plant pathology, problems and progress 1908-
1958. Univ. Wisconsin Press, Madison.
BROADBENT, L. 1964. Control of plant virus diseases, p. 330-364. In M. K. Corbett and
H. Sisler (ed.), Plant virology, Univ. Florida Press, Gainesville.


Vol. 11








Virus Source and Tungro Incidence


IRRI (INTERNATIONAL RICE RESEARCH INSTITUTE). 1973. Annual report for
1972. Los Bafios, Philippines. 246 p.
IRRI (INTERNATIONAL RICE RESEARCH INSTITUTE). 1974. Annual report for
1973. Los Bafios, Philippines. 266 p.
LING, K. C, 1966. Nonpersistence of the tungro virus of rice in its leafhopper vector,
Nephotettix impicticeps. Phytopathology 56:1252-1256.
LING, K. C. 1972. Rice virus diseases. International Rice Research Institute, Los Bafios
Philippines. 142 p.
LING, K. C. 1974. A cage method for studying experimental epidemiology of rice
tungro disease. Phil. Phytopathol. 10: 31-41.
LING, K. C. 1975.i Experimental epidemiology of rice tungro disease I. Effect of some
factors of vector (Nephotettix virescens) on disease incidence. Phil. Phytopathol.
LING, K. C. and M. P. CARBONELL. 1975. Movement of individual viruliferous
Nephotettix virescens in cages and tungro infection of rice seedlings. Phil.
Phytopathol.
MATTHEWS, R. E. F. 1970. Plant virology. Academic Press, New York. 778 p.
MISHRA, M. D., A. GHOSH, F. R. NIAZI, A. N. BASU, and S. P. RAYCHAUDHURI.
1973. The role of graminaceous weeds in the perpetuation of rice tungro virus. J.
Indian Bot. Soc. 52: 176-183.
MIYASHITA, K., Y. ITO, S. YAUSO, T. YAMAGUCHI, and M. ISHII. 1964. Studies of
the dispersal of plant- and leafhoppers. II. Dispersals of Delphacodes striatella
Fallen, Nephotettix cincticeps Uhler, and Deltocepahlus dorsalis Motschulsky
in nursery and paddy field. Jap. J. Ecol. 14: 233-241.
NARAYIrNASAMY, P. 1973. Influence of resistance of rice varieties on their ability to
serve as sources of tungro virus. Madras Agric. J. 60:278-279.
RIVERA, C. T., K. C. LING, and S. H. OU. 1969. Suscept range of rice tungro virus. Phil.
Phytopathol. 5:16-17. (Abstr.)
SCHRODTER, H. and J. ULLRICH. 1966. Weitere Untersunchungen zur Biometeorolo-
gie und Epidemiologie von Phytophthora infestans (Mont.) By Ein neues
Konzept zur L8sung des Problems der epidemiologischen Prognose. Phytopath.
Z. 56: 265-278.
THRESH, J. M. 1971. Temporal pattern of virus spread. Annu. Rev. Phytopathol.
12:111-128.
TING, W. P. 1971. Studies on penyakit merah disease of rice II. Host range of the virus.
Malaysian Agric. J. 48:10-12.
TING, W. P. and C. A. ONG. 1974. Studies of penyakit merah disease of rice IV. Addi-
tional hosts of the virus and its vector. Malaysian Agric. J. 49:269-274.
VAN DER PLANK, J. E. 1975. Principles of plant infection. Academic Press, New York.
216 p.
WATHANAKUL, L. 1964. A study on the host range of tungro and orange leaf viruses of
rice. M. S. Thesis, Univ. Philippines Coll. Agr. 35 p.


Jan. & June, 1975








Phil. Phytopathol. 11:32-45
Jan. & June, 1975
MOVEMENT OF INDIVIDUAL VIRULIFEROUS NEPHOTETTIX VIRESCENS
IN CAGES AND TUNGRO INFECTION OF RICE SEEDLINGS

K. C. LING and M. P. CARBONELL
Plant Pathologist and Research Assistant
The International Rice Research Institute, Los Baiios, Laguna

ABSTRACT

The movements of 1,330 tungro-viruliferous Nephotettix virescens adults were ob-
served hourly from 0800 to 1700 hours after each insect was confined in a screened cage
with seedlings of IR8, which is resistant to the insect, or of Taichung Native 1 (TN1),
which is susceptible. Three types of insect movement related to the spread of tungro
disease were studied seedling-to-seedling, off-seedling, and back-to-seedling.
The percentage of insects that moved varied from one hour to another. The number
of seedling-to-seedling movements per insect was higher than the other two types of
movement.
More insects moved and each individual insect moved more times on the IR8 seed-
lings than on the TN1 seedlings. Consequently, more IR8 seedlings were visited by viru-
liferous insects, but duration of a visit and the interval between movements of each insect
were shorter on IR8 than on TN1 seedlings.
Seedlings of either IR8 or TN1 that became infected were visited for significantly
longer periods by viruliferous insects than those that did not become infected. Hence, the
field resistance of a rice variety to tungro disease could be related to the nonpreference of
the insect for the variety. The low rate of infection of the variety under natural condi-
tions could be due to shorter insect visits to each plant because of frequent movements of
the insect on that variety.


Rice tungro virus is transmitted only
by insect vectors. The rice green leafhop-
per, Nephotetlix virescens (Distant), is
the most efficient of the known vectors.
It walks, jumps, or flies from place to
place to find food, a mate, a comfortable
spot, or a suitable oviposition site; to
escape from an enemy; or to respond to
light or mechanical disturbance. Although
the movement of an insect covers any
change of place, position, or posture, its
movement in relation to a rice plant can
be categorized into one of four types: 1)
a move from one part of the plant to
another; 2) a seedling-to-seedling move,
from one plant to another; 3) an off-
seedling move, from a rice plant to a non-
rice plant location; and 4) a back-to-seed-
ling move, from a nonrice plant location
to a rice plant.
The movements of a viruliferous leaf-


hopper an insect that has had access to
a virus source (Ling, 1972) determine
the spread of the virus disease, although
not every move spreads the disease. If
after exposure to a virus source, an insect
moves on only a single plant for the rest
of its life, it can infect, at most, only that
plant.
The other types of move of a viruli-
ferous insect relate differently to the
spread of the disease. Seedling-to-seedling
moves increase the number of infected
plants. The frequency of such moves may
affect the rate of spread. If viruliferous
insects make only off-seedling movement,
no infection of additional rice plants
occurs. If a rice field is free from virus,
only the incoming viruliferous insects,
moving to rice plants from outside of a
rice field (back-to-seedling) can introduce
the virus to healthy plants. If no viruli-








N. virescens and Tungro of Rice


ferous insects were able to make back-to-
seedling movement, the rice field would
never have the virus disease.
The movement of an insect is initiated
by a stimulus: direct or indirect; physical,
chemical, or biological. Since the stimulus
is complicated and difficult to trace
under uncontrolled conditions, the pre-
sent study only concentrates on the
movement of tungro-viruliferous green
leafhoppers on seedlings of two rice
varieties in relation to the spread of the
rice tungro disease rather than to deter-
mine the cause of the movement.

MATERIALS AND METHODS
Viruliferous insects
The colony of N. virescens used for
this study was reared on rice plants
of Taichung Native 1 (TN1) variety in
screened cages at Los Bafios, Philippines.
The viruliferous adult insects were ob-
tained by confining them with TN1
tungro-diseased plants for 3 to 4 days
before the test.
Seedlings
Rice varieties IR8 and TN1 were
used. IR8 is resistant to the insect while
TN1 is susceptible. Both are susceptible
to the tungro disease though IR8 is
relatively less susceptible than TN1.
Seeds of the two varieties were soaked.
The seedlings were transplanted sepa-
ratedly into 12-cm pots, with 25 seedlings
in five rows per plot. The seedlings were
used for test at 11 to 13 days after soak-
ing the seeds.
Tests
Each pot was placed in the center of a
screened cage, 52 x 52 x 76 cm in dimen-
sions. The cages were kept on benches
under a glass roof. A viruliferous insect
was introduced through an aspirator to
one of. the seedlings in each pot a few


minutes before 0800 hours on each of
the test days and the insects were observ-
ed. If an insect was not on a seedling at
0800 hours, the relevant test was discard-
ed. At the end of each 9-hour test, the
insects were removed and the seedlings
were transferred to a greenhouse to await
development of symptoms of tungro
disease.
To compare insect movement on IR8
and TN1 seedlings, 20 viruliferous insects
were used with each variety for each
day's test. The test was repeated 11
times. Another 23 tests were made on
TN1 seedlings, using 20 to 40 insects per
test, to assess the variation of the move-
ment of the insect on seedlings of a single
variety. A total of 1,330 viruliferous
insects, 6,000 IR8 seedlings, and 27,250
TN1 seedlings were used in the tests be-
tween November 5, 1973 and February
20, 1974.

Observations

After the insects were confined in the
cages, they were observed once every
hour from 0800 to 1700 hours. The in-
sects' whereabouts and the seedlings they
visited were recorded hourly.

Insects that escaped or died before
the test ended were excluded from the
record of insect movement. Similarly,
when a seedling that was visited by
insects died before the period for deve-
lopment of disease symptoms, the data
on all seedlings in that particular pot were
excluded from the seedling infection
record. Data from replicated tests were
averaged.

Insect moves. The insect was consi-
dered to have moved when one of the
three types of movement seedling-to-
seedling, off-seedling, and back-to-seed-


Jan. & June, 1975








Philippine Phytopathology


ling was observed. Only those moves
were recorded. Moves from one part of a"
seedling to another, or from one place to
another outside of the seedlings, were
ignored, as were moves that could have
been made between two observations
from one seedling to others and back, or
off a seedling to a non-seedling site and
back.
The data were compiled on hourly
basis to show the hour-to-hour activities
of the insects, and on the basis of the
whole test period to reveal different
moves made by the insects, and to dis-
close the time interval between two
moves recorded.
Duration of insect visits. Insect visits
refer to the insect stayed on one or more
plants. The data compiled on the basis of
both insect and seedling. The former
refers to the total number of hours that
an insect stayed on a seedling or seed-
lings. The latter refers to the total num-
ber of hours that a seedling was occupied
by the insect in one or more visits.
For convenience in data compilation,
10 visiting hours were considered with
the first observation at 0800 hours and
the final at 1700 hours. Also, unless the
insect moved exactly at the time of obser-
vation, at any particular observation time.
the length of an insect's current visit is
always longer than the time credited; the
length of its visit to the seedling it was on
at the immediately preceding observation
is always shorter than it is credited with.
However, the maximum error in the
duration of insect visits on a seedling
never exceeded 1 hour.

RESULTS'

Movement of tungro-viruliferous.
N. virescens
Percentage of insects that moved. -
The number of viruliferous adults of N.


virescens that moved after being confined
with rice seedlings varied according to the
rice variety (IR8 or TN1) and the length
of confinement. On IR8 seedlings at
hourly intervals, there were 19.6 to
61.1%, with an average of 33.7% of
237 insects that moved. On TN1 seed-
lings, 8.8 to 27.2% moved in a single
hour, with an average of 14.4% of 238
insects. Similar values were obtained with
a test of 1,072 insects on TN1 seedlings
(Fig. 1 and Table 1). The percentage of
insects that moved on IR8 was sig-
nificantly higher than on TN1. Some
insects moved during every hourly in-
terval on IR8 seedlings, the same was
true of insects on TN1. The insects that
moved can be classified according to
types of movement; about 60% moved
from seedlings to seedlings, 30% moved
off seedlings, and 10% moved back to
seedlings. However, hourly averages of
about 64 and 85% of the insects on IR8
and TN1 seedlings, respectively, did not
move with an hour, whether they were
on seedlings or elsewhere.
During the whole test period, 97.9% of
237 viruliferous insects on IR8 seedlings
moved, while only 63.8 and 64.8% of
238 and 1,072 insects, respectively,
moved on TN1 seedlings. The difference
between the moves of the insects on IR8
and TN1 seedlings (97.9 versus 63.8%)
was statistically significant. Of the insects
that moved, some made only one type of
movement, some made two, and others
made all three types of movement. More
insects that moved on IR8 seedlings made
two types of movement seedling-to-
seedling and off-seedling, while more
insects that moved on TN1 seedlings
made only a seedlings-to-seedling move
(Table 2). About 74 and 41% of the
insects on seedlings of IR8 and TN1,
respectively, moved off seedling or back
to seedling, or both.


Vol. 11








N. virescens and Tungro of Rice


Insects (%)
100


/R8
237 insects


TNA
238 insects


80-


0 123456789


0//
012


* Not on seedling
/% Moved off seedling
/ Moved back to seedling
* Moved to another seedling
Z Remained on same seedling


//,6789/
3 4 5 6 7 8 9


0 1 23456789


Time of observation (hours after confinement of insect)


Time of insect moves. Although
some insects had moved on the seedlings
of both rice varieties at every observation,
the percentages of insects that moved
were not the same every hour. The insects
that moved on IR8 seedlings were 30, 61,
56, 35. 23. 25. 31. 20, and 24%!during
the 1st, 2nd, 3rd, 4th, 5th, 6th, 7th, 8th,
and 9th hours following confinement, res-
pectively. The corresponding figures for
the insects on TN1 seedlings were 24, 27,


20, 10, 10, 11, 9, 9, and '10%. On IR8
seedlings, more insects moved during the
second and third hours after confine-
ment, while on TN1 seedlings, more in-
sects moved during the first, second, and
third hours. During and after the fourth
hour, regardless of rice varieties, fewer
insects moved, and the variation among
the percentages of the insects that moved
was small.
Number of moves per insects. The


Table 1. Hourly moves of tungro-viruliferous Nephotettix virescens confined individually
with IR8 or TN1 rice seedlings from 0800 to 1700 hours.

Insects (%) that moved
Insects
tested from seedling off back to
Variety (no.) to seedling seedling seedling Total

IR8 237 21.5 9.6 2.6 33.7
TN1 238 7.7 5.1 1.6 14.4
LSD (1%)1 5.9 3.3 n.s.2 8.9
TN1 1072 9.3 4.5 1.4 15.2

1LSD (1%) for IR8 (237 insects) and TN1 (238 insects).
2 n.s. = not significant at the 5% level.


TNA
/072 insects


Jan. & June, 1975








Philippine Phytopathology


Table 2. Moves of tungro-viruliferous Nephotettix virescens confined individually with
IR8 or TN1 rice seedlings between 0800 and 1700 hours.

Insects (%)

IR8 TN1 TN1
Type of move LSD
(237 (238 (1072
insects) insects) (1%)1 insects)

Insects that did not move 2.1 36.2 28.3 35.2
Insects that moved: 97.9 63.8 28.3 64.8
from seedling to seedling (s-to-s) 23.5 22.7 n.s.2 28.5
s-to-s and off seedling (o-s) 38.5 12.2 13.7 11.4
s-to-s, o-s, and back to seedling 17.2 7.9 n.s. 7.6
off seedling 15.7 16.0 n.s. 12.8
off seedling and back to seedling 3.0 5.0 n.s. 4.5

Insects that moved from seedling to seedling 79.2 42.8 21.6 47.5
Insects that moved off seedling 74.4 41.1 26.1 36.3
Insects that moved back to seedling 20.3 12.9 n.s. 12.1

SLSD (1%) for IR8 (237 insects) and TN1 (238 insects).
2 n.s. = not significant at the 5% level.


move of each insect was observed only
once each hour and thus the maximum
number of moves recorded per hour was
one and the maximum number for the
whole test period was nine. The number
of moves varied with individual insects.
It ranged from 0 through 8, with an
average of 3.03 moves/insect for the in-
sects on IR8 seedlings. On TN1 seedlings,
the number of moves ranged from 0
through 5, with an average of 1.29
moves/insect for the 238 insects; and
from 0 through 9, with an average of 1.37
moves/insect for the 1,072 insects. The
difference in number of moves per in-
sect between the insects on IR8 and TN 1
seedlings (3.03 versus 1.29 moves/insect)
was statistically significant. The dif-
ference in the ranges between the two
samples on TN1 seedlings could be due to
sample size. The frequency distributions
of the insects on IR8 and TN1 seedlings


according to number of moves per insect
were strikingly different (Fig. 2) because
60 and 18% of the insects on IR8 and
TN1 seedlings made three or more moves,
respectively.
The moves were of different types. On
IR8 seedlings, 64% of the moves were
seedling-to-seedling; 28%, off-seedling;
and 8%, back-to-seedling. On TN1 seed-
lings, 53% of the moves were seedling-to-
seedling; 36%, off-seedling; and 11%,
back-to-seedling.
Insects on IR8 seedlings made signi-
ficantly more moves per active insect
than those on TN1 seedlings 3.08 and
1.99, respectively. On IR8 seedlings, the
average number of moves per active insect
was only slightly greater than the average
number of moves per insect for all in-
sects; only a small percentage of the in-
sects did not move at all. In contrast, the
number of moves per active insect on


Vol. 11








N. virescens and Tungro of Rice


Moves (no/insect)
Fig. 2. Number of moves of tungro-viruliferous Nephotettix virescens observed
hourly from 0800 to 1700 hours while the insects were confined with
IR-8 or TN1 seedlings.
TN1 seedlings was about 54% greater (Table 3) was probably due to the gr
than the number of moves per insect for number of insects making an off-see
all insects because 36% of the insects move rather than to a greater numbf
were not seen to move during the entire off-seedling moves per insect.
test period. Interval between twn moves -


eater
dlina
er of

The


The number of seedling-to-seedling
moves per active insect was significantly
greater on IR8 seedlings than on TN1
-seedlings 2.42 and 1.56, respectively.
In other words, the active insects on IR8
seedlings made more seedling-to-seedling
moves. There were 1.15 and 1.10 off-
seedling moves, and 1.14 and 1.08 back-
to-seedling moves per active insect on
seedlings of IR8 and TN1, respectively.
Such differences are not statistically
significant. The significantly higher num-
ber of off-seedling moves per insect from
IR8 seedlings than from TNI seedlings


more moves an insect made in a given
period, the shorter was the average inter-
val between two moves. The shortest
interval was 1 hour and the longest was
9 hours in the present study. Throughout
the test period, 237 insects on IR8 seed-
lings made 717 moves, while 238 and
1,072 insects on TN1 seedlings made 309
and 1,472 moves, respectively. The ave-
rage interval between two moves was
1.85 hours for the insects on IR8 seed-
lings and 2.69 and 2.63 hours for those
on TN1 seedlings. The difference between
1.85 and 2.69 hours was statistically sig-


Jan. & June, 1975








Philippine Phytopathology


Table 3. Moves per tungro-viruliferous insect (Nephotettix virescens) confined individual-
ly with IR8 or TN1 seedlings between 0800 and 1700 hours.

Moves (no./insect)
Variety Insects -
tested Seedling-to- Off- Back-to-
(no.) seedling seedling seedling Total

IR8 237 1.94 0.86 0.23 3.03
TN1 238 0.69 0.46 0.14 1.29
LSD (1%)1 0.54 0.29 n.s.2 0.81
TN1 1072 0.84 0.41 0.12 1.37

1LSD (1%) for IR8 (237 insects) and TN1 (238 insects).
2 n.s = not significant at the 5% level.


nificant at the 5% level. Of moves made
by the insects on IR8 seedlings, 51, 30,
11, 4, 2, 1, and 1% were at intervals of 1.
2, 3, 4, 5, 6, and 7 hours, respectively.
Among insects on TN1 seedling, 37, 27,
13, 7, 6, 2, 1, 6, and 1% of the moves
were at intervals of 1, 2, 3, 4, 5, 6, 7, 8.
and 9 hours, respectively. Thus, most
intervals between two moves were 1 to
2 hours.
Distance of seedling-to-seedling moves
- An insect could move from one seed-
ling to another in any direction and at
any distance in a pot. About 47 and 59%
of seedling-to-seedling moves on seedlings
of IR8 and TN1, respectively, were be-
tween adjacent seedlings. The percentages


of longer moves decreased with increasing
distance (Table 4).

Tungro-viruliferous N. virescens visiting
seedling while encaged

The insects in this study were not al-
ways on seedlings at the hourly observa-
tion times except at the first observation
immediately after confinement; the per-
centage of the insects on seedlings varied
from time to time; insects also differed
in the time they spent with seedlings.
Because some insects visited more than
one seedlings, the time an insect spent
visiting was not equal to the insect's time
per seedling except tor insects which vi-
sited only one seedling during the test


Table 4. Distance traveled (number of seedlings bypassed) in seedling-to-seedling moves
of tungro-viruliferous Nephotettix virescens confined with IR8 or TN1 rice
seedlings.

Moves (%)
Insects
tested Moves No. of seedlings bypassed
Variety (no.) (no.) 0 1 2 3

IR8 237 458 47.5 33.2 12.6 6.7
TN1 238 166 59.4 25.3 12.6 2.7
TN1 1072 914 54.9 30.7 10.1 4.3


Vol. 11








N. virescens and Tungro of Rice


period. The duration of the average in-
sect visit per seedling was not equal to
the total time that insects spent on seed-
lings divided by the total number of seed-
lings visited by insects because of the
variation in the number of insects in the
replicated trials. Furthermore, because
some insects moved back to seedlings
they had already visited, the number of
seedlings visited by an insect was not
equal to the number of its seedling-to-
seedling moves.
All insects were on seedlings at the be-
ginning of the test; some stayed; others
moved off. Some that moved off later
moved back to seedlings; others did not.
The percentage of insects on seedlings of
both rice varieties decreased gradually
as the time of confinement of the insects
increased (Fig. 1). However, more insects
confined with TN1 seedlings stayed on
the seedlings than did those confined
with IR8 seedlings (Table 5). Therefore,
the time spent visiting seedlings by the
insects with TN1 seedlings (7.94 hours/
insect) became significantly longer than
that of insects with IR8 seedlings (6.37
hours/insect) (Table 5).
Since the frequencies of the insect
moves on seedlings of IR8 and TN1 were
different (Table 3), the number of seed-
lings visited by an insect with IR8 seed-


lings was significantly higher than that of
an insect with TN1 seedlings (3.09 versus
1.78 seedlings/insect). Hence more IR8
seedlings were visited by insects in the
test period. Because the insects paid
shorter visits to IR8 seedlings and visited
more of them, the insect's visiting time
per IR8 seedling was significantly shorter
than for TN1 (2.09 versus 4.82 hours/
seedling) (Table 5).

Seedlings infected by tungro-viruliferous
N. virescens
Seedling infection. In each test, 25
seedlings were exposed to each viruli-
ferous insect, but only one to eight seed-
lings of both rice varieties were visited
by the insect. Ten percent of the IR8
seedlings visited and 46.3% of the TN1
seedlings visited became infected (Table
6).
The number of infected seedlings per
insect varied among replicates, possibly
because of a variation in the sources of
viruliferous insects for the various tests.
However, the average number infected
was significantly greater among TN1
seedlings (0.70 infected seedling/insect)
than among IR8 seedlings (0.27 infected
seedling/insect), mainly because more in-
sects on the TN1 seedlings were in-
fective. An infective insect is an in-


Table 5. Visits of tungro-viruliferous Nephotettix virescens to IR8 or TN1 rice seedlings.

Insects on
seedlings Insects Seedling
Insects at hourly visits to Seedlings visits from
tested observations seedlings visited insects
Variety (no.) (%) (hr/insect) (no./insect) (hr/seedling)

IR8 237 63.7 6.37 3.09 2.09
TN1 238 79.4 7.94 1.78 4.82
LSD (1%)' 15.7 1.57 0.61 1.96
TN1 1072 81.5 8.15 1.90 4.62

1 LSD (1%) for IR8 (237 insects) and TN1 (238 insects).


Jan. & June, 1975








Philippine Phytopathology


Table 6. Rice seedlings visits from tungro-viruliferous Nephotettix virescens and seedling
infection.

Seedlings infected
Time on
Insects Seedlings (no./in- seedling
tested visited (no./ fective (hr/seedling
Variety (no.) (no.) (%) insect) insect) infected)

IR8 237 718 10.0 0.27 1.17 3.30
TN1 238 415 46.3 0.70 1.23 6.18
LSD (1%)1 19.7 0.27 n.s.2 2.33
TN1 1072 1660 40.9 0.60 1.17 6.62

SLSD (1%) for IR8 (237 insects) and TN1 (238 insects).
2 n.s. = not significant at the 5% level.

sect that actually transmits the disease 3.13 seedlings. Infective insects visited
during the test period (Ling, 1972). one to five TN1 seedlings in the same
Not every seedling visited by an in- test period, but most visited only one or
fective insect became infected because two seedlings; the average infective in-
the transmission of the tungro virus by sect visited 1.78 seedlings. Only one to
N. virescens fluctuates hourly (Ling, three of the visited IR8 seedlings became
1969a). Overall, the infective insects vi- infected, whereas one to four of the vi-
sited one to six IR8 seedlings in the test sited TN1 seedlings became infected.
period; most visited two to four seedlings; Usually, however, only one of the visited
on the average, an infective insect visited seedlings of either variety became in-
Table 7. Relation of number of rice seedlings infected to number visited by tungro-
viruliferous Nephotettix virescens in 9 hours.

Frequency distribution (%)
Seedlings Seedlings visited (no./insect)
infected
(no.) 1 2 3 4 5 6 Total

IR8 (52 infective insects)
1 9.6 21.2 19.2 19.2 11.6 1.9 82.7
2 5.8 1.9 3.9 0 1.9 13.5
3 1.9 1.9 0 0 3.8
Total 9.6 27.0 23.0 25.0 11.6 3.8 100.0

TN1 (459 infective insects)
1 53.8 18.3 7.8 3.3 0.9 0 84.1
2 7.0 4.6 1.3 1.3 0 14.2
3 0.2 0.9 0.4 0 1.5
4 0.2 0 0 0.2
Total 53.8 25.3 12.6 5.7 2.6 0 100.0


Vol. 11








N. virescens and Tungro of Rice


fected (Table 7); the averages were
similar 1.17 and 1.23 infected seed-
lings/infective insect for the insects on
seedlings of IR8 and TN1, respectively
(Table 6). Infective insects transmitted
the tungro virus almost as efficiently on
IR8 seedlings as on TN1 seedlings.

Of the 511 infective insects studied,
only one infected as many as four TN1
seedlings with tungro in a 9-hour period
(Table 7).

The timing of the seedlings' visits from
insects varied. The time of the first visit
of any insect to a particular seedling that
later developed tungro symptoms ranged
from 0 to the 7th hour after confinement
for IR8 and from 0 to the 9th hour for
TN1. The frequency distribution of the
IR8 seedlings that became infected rank-
ed according to the time of the first
insect visit indicated that 42, 4, 10, 18,
1, 16, 3, and 6% of the seedlings infected
were first visited at 0, 1, 2, 3, 4, 5, 6, and
7 hours after confinement, respectively.
Sixty percent of the TN1 seedlings infect-
ed were first visited by the insects im-
mediately after the insects were confined
with the seedlings. The other 40% of the
seedlings infected received their first visits
later but the percentage decreased gra-
dually as the period after the confine-
ment of the insects lengthened.

Duration of insect visits and infection.
- The duration of insect visits to seed-
lings infected ranged between 1 and 10
hours. However, the insect visits to TN1
seedlings infected were significantly longer
than those to IR8 seedlings (6.18 versus
3.30 hours/seedling infected) (Table 6),
and apparently this difference affected
the difference between the two varieties
in percentage of seedlings infected. How-
ever, the difference between these two
varieties in susceptibility to tungro
disease cannot be discounted.


A comparison of the duration of
insect visits to subsequently infected and
uninfected seedlings of the same variety
illustrated that the average duration of
visit of a viruliferous insect or an infec-
tive insect to a seedling that became
infected was significantly longer than
that to a seedling that did not become
infected, regardless of the tested variety
(Table 8).


DISCUSSION

Although N. virescens' can move in
various ways, strictly speaking only two
kinds of move are related to the spread
of tungro disease in a rice field. The first
move may be from anywhere to diseased
plants, where the insects feed and become
viruliferous. In the second move, the viru-
liferous insects move from anywhere to
healthy plants, and introduce the virus
by feeding. This paper discusses only the
second movement.

After the viruliferous insects had been
confined individually with rice seedlings,
their whereabouts were monitored hour-
ly. Although the results of this study can-
not illustrate the movement of viruli-
ferous N. virescens under natural condi-
tions, since cage size limited the distance
the insects could travel, the cage method
has advantages. It makes the insects easier
to locate; and the observation disturbs
the insects less than when their move-
ments are studied in a rice field. The me-
thod can be used to study the differences
in movement among insects on seedlings
of different rice varieties, and to deter.
mine the effect of environmental condi-
tions on the movement of the insect.

An insect's movement from one place
to another is initiated by a stimulus, but
is conditioned by intrinsic factors and in-


Jan. & June, 1975








Philippine Phytopathology


Table 8. Average duration of visits to tungro-infected and uninfected IR8 or TN1 rice
seedlings from viruliferous and infective Nephotettix virescens.

IR8 TN1

Insect Insect
time on time on
Seedlings seedling Seedlings seedling
Seedling (no.) (hr/seedling) (no.) (hr/seedling)

Viruliferous insects1
Infected 63 3.30 541 6.62
Uninfected 655 1.93 1119 3.62
LSD (1%) 1.32 1.14

Infective insects2
Infected 63 3.67 541 6.90
Uninfected 100 1.86 275 1.78
LSD (1%) 0.91 0.42

SAverage of 12 trials for IR8, 35 trials for TN1.
2 Average of 52 insects for IR8, 459 insects for TNT.


fluenced by environmental agents. Move-
ment varies among individual insects and
from time to time. Although the reasons
for the movement of the viruliferous
N. virescens in this study are not clear,
there were always some insects that
moved at hourly intervals. The fact that a
large percentage of the insects moved
during the first 3 hours after confinement
may be related directly to the time of the
day, because the tests in the present
study were always started at 0800 hours
Naito and Masaki (1967) found that
probing by N. cincticeps was not pro-
nounced in the forenoon and midnight.
but increased markedly from about
1800 to 2100 hours. If the movement ol
an insect is negatively correlated to itv
probing activity, it may be related to the
time of day.
Viruliferous N. virescens were initially
confined on seedlings in the present
study, but some later moved off to non-
seedling sites. On the average, 81.5% of


1,072 insects were observed hourly on
TN1 seedlings (Table 5) during the 9-hour
test period. Assuming the insects fed on
rice seedlings when they were on them,
the figure correlates closely with the con-
clusion of Naito and Masaki (1967)
that the average of total feeding time of
N. cincticeps in a given period was
84.9%.

Viruliferous N. virescens should be
capable of travelling farther than 25 cm,
but that was about the maximum dis-
tance an insect moved in the present
study; undoubtedly the size of the cage
limited movement. Miyashita et al.
(1964) reported that the maximum dis-
tance N. cincticeps moved in one day
was 7 to 13 m in the nursery and 23 to
41 m in the field. If the off-seedling and
back-to-seedling moves are considered
long-distance moves, more insects on IR8
seedlings than on TN1 seedlings made
long-distance moves.


Vol. 11








N. virescens and Tungro of Rice


Although the movement of the in-
sects varied among individuals and from
time to time, there were no marked dif.
ferences in the movements based on
studies of the 238 insects or 1,072 in-
sects on TN1 seedlings. However, more
insects moved on IR8 seedlings than on
TN1 seedlings (Table 1); the insects made
more moves on IR8 seedlings (Fig. 2),
particularly seedling-to-seedling moves
(Table 3); and the insects stayed for
shorter periods on IR8 seedlings
(Table 5). Those fundamental differences
meant that insects on IR8 seedlings
moved at shorter intervals, visiting more
IR8 seedlings per insect (Table 5), and
stayed on each IR8 seedling for a shorter
time (Table 5). Although the real mech-
anism of the effect of rice varieties
on the movement of viruliferous N. vi-
rescens remains to be determined, the
differences in the moves of the insects on
seedlings of the two rice varieties are
probably due to the differences in the
insect's preference for one variety over
another.

Painter (1951) used the terms "pre-
ference" and "nonpreference" to denote
the group of plant characters and insect
responses that lead to or away from the
use of a particular plant or variety for
oviposition, for food, for shelter, or for
combinations of the three purposes. Fre-
quent movement, therefore, can be con-
sidered associated with restlessness of the
insects resulting upon contact with the
nonpreferred host. It is highly possible
that tungro-viruliferous N. virescens has a
greater preference for TN1 seedlings than
for IR8 seedlings because of more fre-
quent movement of the insects on IR8
seedlings than on TN1 seedlings.
The nonpreference for IR8 seedlings
by N. virescens is a part of the resistance
mechanism of the rice variety to the in-
sect. IR8 seedlings have been known to
be resistant to N. virescens because of the


insect's shorter life span on the seedlings
(Ling, 1969b; Cheng and Pathak, 1972).
The mechanism of this plant resistance is
"antibiosis" which is a term proposed by
Painter (1941, 1951) to describe the ad-
verse effects on an insect which result
when the insect teeds on a resistant host-
plant variety. Therefore, IR8 seedlings
not only are not preferred by but also
exhibit antibiosis in relation to N. vires-
cens. It is uncertain whether these me-
chanisms result from independent genetic
characters.
All transmission of plant virus diseases
by arthropods depends on the movement
of insects from one host to another (Car-
ter, 1973). However, the rate at which a
virus disease spreads may not always be
directly proportional to the frequency of
movement of viruliferous insects from
one plant to another. For instance, when
the interval between two moves is shorter
than the inoculation threshold period -
the minimum time necessary for a vector
to feed on a test plant in order to trans-
mit the virus (Federation of British Plant
Pathologists, 1973) the frequency of
movements could be very high, but not a
single plant would be infected.
The viruliferous N. virescens moved
more frequently on seedlings of IR8, thus
visiting more IR8 seedlings than TN1
seedlings (Table 5), but the percentage of
infected seedlings of IR8 was much lower
than that of TN1 (Table 6). It is difficult
to conclude that the low percentage of
infected seedlings of IR8 was due to
shorter duration of insect visits per seed-
ling (Table 6) because of lack of evidence
that the low percentage of infected seed-
lings of IR8 was not due to less suscepti-
bility of the variety to tungro disease.
Nevertheless, a comparison of the dura-
tion of visits of either a viruliferous insect
or an infective insect to seedlings of
either IR8 or TN1 that became infected
and those uninfected indicated that the


Jan. & June, 1975








Philippine Phytopathology


visits were always longer in case of the
former compared to the latter (Table 8).
Therefore, the rate of spread of tungro
disease is determined not only by the
frequency of the movement of the viruli-
ferous N. virescens but also by the dura-
tion of the insect's visit to a seedling.

It is known that the percentage of
infected seedlings increases gradually as
the inoculation access time lengthens
from 1 to 9 hours (IRRI, 1973). Inocula-
tion access time is the length of time a
vector is allowed to spend on a test host
in a transmission experiment (Federa-
tion of British Plant Pathologists, 1973).
In this paper, the duration of an insect's
visit to a seedling is considered equivalent
to the inoculation access time. Hence,
peddling infection should be increased
by lengthening the insect's visit. This ex-
plains why the infected seedlings of either
IR8 or TN1 had a longer duration of in-
sect visits per seedling.

Robinson (1969) defined the field
resistance as "any resistance which in-
fluences the epidemic (i.e. in the field)
but which is not immediately apparent
in the laboratory, glasshouse or breeder's


plot (i.e. not in the field)." When IR8
seedlings were tested by mass screening,
they often showed a susceptible reaction
to tungro disease (Ling, 1969 b, c), but
in the field IR8 often showed resistance
(Thailand Ministry of Agriculture, 1966),
particularly in the early years when the
insect had not adapted to the variety.
The field resistance of IR8 to tungro
disease was definitely related to its re-
sistance to N. virescens, but that might
not be entirely due to antibiosis, parti-
cularly when the variety was tested toge-
ther with other varieties for their resis-
tance to tungro disease by natural infec-
tion in a field. The field resistance of IR8
may be due to the green leafhoppers free
movement in a field, which resulted in
shorter visits to each plant. In the green-
house where insect movement is some-
what restricted, an insect's visit to a seed-
ling is longer.
The present results suggest that one
reason for a rice variety's field resistance
to tungro disease could be related to the
shorter visits to each plant by viruliferous
insects. That could be due to the frequent
movement of the insects, caused by their
nonpreference for the variety.


LITERATURE CITED

CARTER, W. 1973. Insects in relation to plant disease, 2nd ed. John Wiley & Sons,
New York. 759 p.
CHENG, C. H. and M. D. PATHAK. 1972. Resistance to Nephotettix virescens. J. Econ.
Entomol. 65:1148-1153.
FEDERATION OF BRITISH PLANT PATHOLOGISTS. 1973. A guide to the use of
terms in plant pathology. Phytopath. Pap. no. 17, 55 p.
IRRI (INTERNATIONAL RICE RESEARCH INSTITUTE). 1973. Annual report for
1972. Los Bafios, Philippines. 238 p.
LING, K. C. 1969a. Nonpropagative leafhopper-borne viruses, p. 257-277. In K. Mara-
morosch (ed.) Viruses, vectors, and vegetation. Inter-science Publishers, New York.
LING, K. C. 1969b. Preliminary studies on the resistance of rice varieties to tungro and
its vector, Nephotettix impicticeps. Phil. Phytopathol. 5:7. (Abstr.)
LING, K. C. 1969c. Testing rice varieties for resistance to tungro disease, p. 277-291.
In The virus diseases of the rice plant, Proceedings of a symposium at International
Rice Research Institute, April 1967. Johns Hopkins Press, Baltimore.
LING, K. C. 1972. Rice virus diseases. International Rice Research Institute, Los Bafios,
Philippines. 142 p.


Vol. 11








Jan. & June, 1975


N. virescens'and Tungro of Rice


MIYASHITA, K., Y. ITO, S. YASUO, T. YAMAGUCHI, and M. ISHII. 1964. Studies on
the dispersal of plant- and leafhoppers. II. Dispersals of Delphacodes striatella
Fallen, Nephotettix cincticeps Uhler, and Deltocephalus dorsalis Motschulsky in
nursery and paddy field. Jap. J. Ecol. 14:233-241.
NAITO, A. and J. MASAKI. 1967. Studies on the feeding behavior of green rice leaf-
hopper, Nephotettix cincticeps Uhler. Probing frequency of the audit leafhopper
(in Japanese, English summary). J. Apol. Entomol. Zool. 11: 150-156.
FAINTER, R. H. 1941. The economic value and biologic significance of plant resistance
to insect attack. J. Econ. Entomol. 34:358-367.
PAINTER, R. H. 1951. Insect resistance in crop plants. MacMillan Co., New York. 520 p.
ROBINSON, R. A. 1969. Disease resistance terminology. Rev. Appl. Mycol. 48:593-606.
THAILAND MINISTRY OF AGRICULTURE, Rice Department, Breeding Division.
1966. Notes on reactions of rice varieties to tungro-like virus disease in 1965. 41 p.
(mimeo)








Phil. Phytopathol. 11:46-57
Jan. & June, 1975
EFFECT OF TEMPERATURE ON THE TRANSMISSION OF
RICE TUNGRO VIRUS BY NEPHOTETTIX VIRESCENS

K. C. LING and E. R. TIONGCO
Plant Pathologist and Research Assistant

The International Rice Research Institute, Los Baios, Laguna

The authors wish to acknowledge the use of the facilities of the IRRI Phytotron,
donated by the Australian government, for the present study. They are grateful to Dr.
Kwanchai A. Gomez for her statistical assistance and to Messrs. Florencio Salazar and
Ireneo Juinio for their technical help.
ABSTRACT

The effect of temperature on the transmission of rice tungro virus by adult green
leafhopper, Nephotettix virescens (Distant) was studied under controlled conditions. The
insects acquired the virus from diseased plants and inoculated rice seedlings at tem-
peratures ranging from 10 to 380C. The transmission efficiency tended to increase with
increasing temperature from 10 to 310C.
In tests at 13, 20, 27, and 340C, the infective capacity proved to be highest at 34 C.
The life span of tungro-viruliferous insects increased as temperatures decreased from 34
to 130C. The longest retention periods at 13 and 320C were 22 and 6 days, respectively,
after an acquisition feeding at room temperature. At 70C, low insect's infectivity was re-
corded but the low temperature neither increased the infectivity, nor altered the charac-
ter of gradual loss of the infectivity.


The transmission of a virus from an
infected plant to a healthy plant by an
insect vector involves the virus, the vec-
tor, the plant, and environmental condi-
tions. As temperature affects various as-
pects of the virus, the insect vector, and
the host plant it should also affect the
transmission of a plant virus by an insect.
Six steps occur in the transmission of
a virus from an infected plant to a heal-
thy plant by an insect vector: 1) a virus-
free insect moves to a diseased plant,
2) The insect feeds (acquisition feeding)
and' acquires the virus, becoming viruli-
ferous (Ling, 1972). The length of time
that a test vector has access to a virus
source in transmission tests is termed
"acquisition access time" (Federation of
British Plant Pathologists, 1973); 3) After
a latent period, if any, the insect becomes
capable to infect a plant; 4) The insect
moves to a healthy plant; 5) The viruli-
ferous insect feeds on the healthy plant


(inoculation feeding), inoculating it with
the virus. The time that a vector is allow-
ed to spend on a test host in transmission
experiments is known as the "inoculation
access time" (Federation of British Plant
Pathologists, 1973); 6) The inoculated
plant develops disease symptoms. With
these six steps, transmission is completed,
and the insect is considered infective be-
cause it actually transmits the disease
(Ling, 1972).
Several other features of the virus-
vector interaction, such as retention of
the virus by the vector, transstadial pas-
sage, transovarial passage, and infective
capacity (Ling, 1974) are also related to
the transmission of the virus to the host
plant by the insect.

The effect of temperature on the
transmission should be examined at each
of the six transmission steps. In transmis-
sion experiments, however, an insect is








Transmission of Rice Tungro Virus


transferred artificially to a plant in steps
1 and 4. Step 3 is omitted in study of the
rice tungro virus because the virus has no
demonstrable latent period in the rice
green leafhopper, Nephotettix virescens
(Distant) (Ling, 1966). For transmission
study, step 6, symptom development of
inoculated plants at a uniform favorable
temperature is desired.
This paper reports the effect of tem-
perature on acquisition feeding, inocula-
tion feeding, infection of rice seedlings,
infective capacity, and retention of the
tungro virus by the insect vectors.

MATERIALS AND METHODS

Adult N. virescens insects used in this
study were reared on Taichung Native 1
(TNI rice plants in screened cages at
Los Bafios, Philippines. To make the
insects viruliferous, they were confined
on tungro-diseased TN1 plants for 4 days
unless otherwise specified.

To test the insects' infectivity healthy
1-week-old TN1 seedlings were placed
individually into 15 x 180 mm test tubes
containing a small amount of water. One
viruliferous insect was transferred with an
aspirator to each seedling and the tube
was immediately covered with a polypro-
pylene culture cap. After an inoculation
access time (usually 24 hours) the insects
were either discarded or transferred to
other seedlings. The inoculated seedlings
were transplanted into pots and kept in
the greenhouse for disease symptoms to
develop.

Seedling infection was used, as the
criterion for evaluating whether an insect
acquired the virus from diseased plants or
a viruliferous insect inoculated a seedling
at a given ambient temperature. The in-
fected seedling served as evidence that the
insect had performed the task at the given
temperature. Because there was always


one insect per seedling in the test, the
percentage of infective insects was identi-
cal to the percentage of infected seed-
lings. Either percentage indicates differen-
ces in transmission among treatments.

Insects, seedlings in test tubes, and
diseased plants in cages were kept at the
desired temperature for 10 to 30 minutes
before initiating an experiment. The
experiments were conducted in the IRRI
Phytotron using Koitotron KB-10D
growth cabinets, classrooms, or dark
rooms with light, where the temperatures
were regulated.
Acquisition feeding

To determine the effect of tempera-
ture on the acquisition of tungro virus by
N. virescens, the insects were confined on
diseased TN1 plants in screened cages at
different temperatures (10, 13, 16, 20,
25, 31, and 380C) for 1 to 4 days. After
the acquisition feeding, 4,649 insects
were tested for their infectivity, using an
inoculation access time of 24 hours 'at
room temperature (26-310C).


Inoculation feeding
To determine the effect of tempera-
ture on inoculation feeding, after acquisi-
tion feeding in the greenhouse, insects
were transferred, one to a seedling, to
seedlings in test tubes at different tempe-
ratures (10, 13, 16, 20, 25, 31, and 380C)
for an inoculation access time of 24 hours
hours. There were 1,201 insects tested.
Infection of rice seedlings
To investigate the infection of rice
seedlings at different temperature ranges,
180 virus-free N. virescens adults per cage
were introduced and confined for 7 days
in cages containing either one pot of
diseased rice plants and 15 pots of heal-
thy seedlings or four pots of diseased
plants and 12 pots of healthy seedlings.


Jan. & June, 1975









Philippine Phytopathology


Each pot contained 25 TN1 seedlings.
The cages were kept in classrooms
with 12-hour day/12-hour night tempera-
tures of 24/16, 27/19, and 30/22aC. At
the end of the test, the insects were re-
moved and all seedlings were transferred
to the greenhouse for symptoms to de-
velop. The test was repeated four times
and involved 5,400 insects, 75 pots of
diseased plants, and 9,232 healthy TN1
seedlings in 405 pots, excluding seedlings
that died before symptoms developed.

Infective capacity

To determine the infective capacity of
N. virescens, 20 viruliferous adults were
transferred individually to a succession of
healthy seedlings. The transfers at 30-
minute intervals were made from 0800 to
1800 hours at temperatures of 13, 20, 27,
or 340C. Insects that died or escaped
during the test were replaced with other
viruliferous insects that had been kept
separately on healthy seedlings at test
temperature since the start of the test.
The test was repeated three times and
involved 6,400 seedlings exposed to 320
viruliferous insects.

Life span

To study the life span of adult tungro-
viruliferous green leafhoppers at different
constant temperatures, insect pairs (one
female and one male) were confined in
test tubes. Each tube contained three
healthy TN1 seedlings and was maintain-
ed at 13, 20, 27, or 340C. Eighty tubes
and 160 insects were used for each
treatment.

The insects in the test tubes were
examined once each morning, dead in-
sects were counted and their life spans
recorded. Surviving insects were trans-
ferred to fresh seedlings at 2- or 3-day
intervals, because seedlings kept for


several days in tubes, particularly at the
higher temperatures, tended to become
chlorotic. The test was repeated once
using 1,280 insects.

Retention period
The retention period of a plant virus
in an insect vector is generally determined
by serial transmission at certain time
intervals (i.e., hourly, daily) following an
acquisition feeding. When the serial trans-
mission is conducted at a low tempera-
ture (7 or 130C) and transmission does
not occur, the reason could be either that
the insect is not infective or that it could
not inoculate the seedling at such low
temperature. This could cause a misinter-
pretation of results.

The length of retention of the tungro
virus by N. virescens were tested by sub-
jecting a colony of viruliferous insects to
two temperature regimes, and sampling
daily to determine how much of the
colony continued to be infective. The in-
sects were placed in test tubes (10 insects
in each), containing five healthy TN1
seedlings. The tubes were kept in Koito-
tron KB-10D growth cabinets with some
maintained at 130C and some at 320C.
Eighty insects were withdrawn from each
temperature regime daily and their infec-
tivity tested with an inoculation access
time of 24 hours at 320C. The insects
were then examined and discarded, and
seedlings from tubes with dead insects
were discarded. The daily infectivity tests
continued until the insect supply was
exhausted. Because seedlings in the test
tubes would have become infected in a
few days and could have introduced error
into the retention test by reinfecting
insects that had lost their infectivity, the
seedlings were replaced daily. The reten-
tion test was performed three times,
involving 4,378 insects at 130C and 2,517
insects at 320C.


Vol. 11









Transmission of Rice Tungro Virus


To verify the retention of infectivity
at a low temperature, viruliferous insects
were subjected to daily serial transmission
tests for 3 consecutive days at two tem-
perature regimes. Four batches of insects
each received one of four treatments: 1)
room temperature (26-310C) throughout
the test period of 3 days (control); 2)
room temperature on the first day; 70C
on the second day, and room temperature
on the third day; 3) 70C on the first day
and room temperature on the second and
third days; or 4) 70C on the first and
second days and room temperature on
the third day. The experiment was carried
out twice, and used a total of 468 insects
at 106 to 133 insects per treatment.

RESULTS
Acquisition feeding

The rice green leafhopper acquired the


tungro virus from diseased plants at all
temperatures (10, 13, 16, 20, 25, 31, and
380C). However, the percentage of the
insects that acquired the virus and be-
come infective varied with temperature.
Based on the mean of four acquisition ac-
cess times studied, the lowest percentage
of infective insects recorded, about 7o ,
occurred at 100C and the highest, 85%
occurred at 310C. The differences in per-
centage of infective insects between 25
and 380C were not great (Fig. 1).
The percentage of infective insects was
affected by the length of acquisition
access time. Overall averages of 55, 58,
62, and 66% of infective insects were ob-
tained from acquisition access times of
1, 2, 3, and 4 days, respectively.
Inoculation feeding

After acquisition feeding in the green-
house, the insects inoculated the seedlings


Infective insects (%)
1001


80-


Inoculation




'Acquisition


I I


10 13 16 20 25
Temperature (OC)


Fig. 1. Effect of temperature on the transmission (acquisition and inoculation)
of the rice tungro virus by Nephotettix virescens.


60 F


40



20


.


! !


Jan. & June, 1975









Philippine Phytopathology


at all test temperatures, given an inocula-
tion access time of 24 hours. However,
the percentage of seedlings infected
varied with temperature, with the lowest,
about 12%, occurring at 100C and the
highest, about 91%, occurring at 310C.
Only a slight difference occurred between
20 and 380C (Fig. 1).

Infection of rice seedlings

The difference in the infection of rice
seedlings at various temperatures indi-
cates the effect of temperature on the
acquisition feeding, the inoculation feed-
ing, and the movement of the insects
within the cages. The infected seedlings
served as evidence that the virus-free in-
sects became viruliferous and transmitted
the tungro virus from the diseased plants
to healthy seedlings in the cages at day/
night temperatures of 24/16, 27/19, and
30/220C (Table 1). However, the per-
centage of seedlings infected was affected
by both the temperature and the amount
of the virus source in the cage.


When the virus source was one pot of
diseased plants in a 16-pot cage, the per-
centage of seedlings infected increased as
the day/night temperatures were in-
creased. However, the only statistically
significant difference in the percentage of
.seedlings infected occurred between 24/
160C (33.5%) and 30/220C (43.1%)
(Table 1).

When the virus source was increased to
four pots of diseased plants in a 16-pot
cage, percentage of seedlings infected in
increased as the day/night temperatures
increased. However, the differences in
the percentage of seedlings infected
among the three day/night temperature
regimes were not significant. The percent-
age of seedlings infected was always high-
er when virus source increased regardless
of the day/night temperature regime
(Table 1).
Infective capacity
Of 320 viruliferous insects tested for
their infective capacities at four tempera-


Table 1. Effect of temperature and virus source availability on spreading of tungro virus
by Nephotettix virescens in a susceptible rice variety (Taichung Native 1)1

Virus source

1 pot diseased plants 4 pots diseased plants

Day/night Seedlings Seedlings Seedlings Seedlings
temperature2 tested infected tested infected
(C) (no.) (%)3 (no.) (%)3

24/16 1717 33.5 a 1375 61.9 a
27/19 1682 37.1 ab 1382 67.4 a
30/22 1716 43.1 b 1360 72.5 a

1180 virus-free insects and 15 or 12 pots of healthy TN1 seedlings were
caged for 7 days respectively with 1 or 4 pots of diseased plants.
212-hour day/12-hour night.
3 For each column, means followed by a common letter are not significant-
ly different at the 5% level.


Vol. 11








Transmission of Rice Tungro Virus


tures, 296, including 66 replacement in-
sects, infected 31% of the 5,547 seed-
lings, excluding 373 seedlings that died
before disease symptoms developed.

Both average and maximum percent-
ages of infected seedlings increased as the
temperature increased, with the highest
(38.5% for the average and 84.2% for the
maximum) occurring at 340C (Table 2).
When the percentages of infected seed-
lings were converted to the infective capa-
city by assuming that the infectivity of
the insects during the test period of 10
hours remained unchanged for the rest of


the 24-hour day, the infective capacities
were 8.5, 15.3, 15.8, and 18.5 infected
seedlings/infective insect/day, on an aver-
age, and 31.2, 36.0, 37.3, and 40.4 in-
fected seedlings/infective insect/day,
maximum, at 13, 20, 27, and 340C, res-
pectively.
Life span
The average and maximum life span of
individual tungro-viruliferous adults ofN.
virescens increased as temperature de-
creased from 34 to 130C (Table 3). The
longest life span of the insects, at 130C,
was 118 days, 100 days longer than that


Table 2. Effect of temperature on the infective capacity of tungro-infective Nephotettix
virescens.1

Temper- Infective Seedlings Seedlings Infected seedlings (%)
nature insects survived infected
(C) (no.) (no.) (no.) Min. Mean2 Max.

13 62 1146 203 5.0 17.7 a 65.0
20 77 1442 462 5.0 31.8 b 75.0
27 79 1487 488 5.0 32.9 bc 77.8
34 78 1472 565 5.0 38.5 c 84.2

1 For 10 hours, the insects, one per seedling, were transferred at 30-minute intervals to
fresh Taichung Native 1 seedlings.
2 Means followed by a common letter are not significantly different at the 5% level.



Table 3: Life span of tungro-viruliferous adults of Nephotettix virescens at different
constant temperatures.

Life span (days)
Temperature Insects tested
(C) (no.) Longest Average1

13 320 118 34.2 d
20 320 76 18.5 c
27 320 28 13.0 b
34 320 18 8.2 a

'Means followed by a common letter are not significantly different at the 1% level.


Jan. & June, 1975








Philippine Phytopathology


at 34uC. The life span of female and male
viruliferous adults ofN. virescens did not
differ significantly.

Retention period
Retention at 13 and 320C The re-
tention period of the tungro virus by N.
virescens adults at 130C differed from
that at 320C. The longest retention
period was 22 days after an acquisition
feeding for the insects that were kept at
130C and 6 days for the insects kept at
320C. However, at both temperatures the
infectivity decreased gradually with time
after the acquisition feeding (Fig. 2)
Retention at 70C. Four treatments
(Fig. 3) were used to determine the re-
tention of infectivity of N. virescens at
7C.
Treatment I (control). When the
insects were kept at room temperature
(26-310C), the percentage of infective
insects decreased drastically between the
first and third day after acquisition
feeding. This confirmed a previous study
(Ling, 1966).
Treatment 2. The percentage of infec-
tive insects kept at room temperature for
the first day after acquisition feeding was
similar to that of the control insects
(Treatment 1). When the insects were
moved to a 70C environment on the
second day, the percentage of infective
insects became low. When the insects
were transferred back to room tempera-
ture on the third day, the percentage of
infective insects was higher than that at
70C on the second day and higher than
that of the control insects on the third
day. Apparently, the infectivity of the
insects was retained at 70C on the second
day.
Treatment 3. Insects kept at 70C on
the first day after acquisition feeding had
low infectivity. When the insects were
transferred to room temperature on the
second and third days, the percentages of


infective insects became much higher
than that of the control insects on the
second and third days. That demonstrat-
ed the lower rate at which infectivity was
lost at 70C. However, the infectivity on
the third day was much lower than on the
second day, revealing again the loss of in-
fectivity with time at room temperature.
Treatment 4. The infectivity of the
insects kept at 7C for the first two days
was low. When the insects were transfer-
red to room temperature on the third
day, the percentage of infective insects
was much higher than that of the insects
on the other three treatments on the
third day. This again showed that the loss
of infectivity occurred at a much lower
rate at 70C even for two consecutive
days.

DISCUSSION

Temperature is one climatic factor
that affects directly the development (Ya-
mamoto and Suenaga, 1956), movement
(Akino, 1969) and probing (Naito and
Masaki, 1967; Oya and Sato, 1973) of the
rice green leafhopper, Nephotettix cincti-
ceps, which is a vector of several rice virus
and virus-like diseases. The present study
revealed that tungro-viruliferous N. vires-
cens lives longer at a lower temperature
(Table 3). The aphid Cepitophorus fragae-
folii, a vector of strawberry yellows virus
complex (Miller, 1952), as well as Am-
phorophora lactucae, a vector of sow-
thistle yellow vein virus (Duffus, 1963),
have been found to follow a similar pat-
tern. Also, temperature affects the virus
in the host plant. For instance, the move-
ment of rice stripe virus in rice plants was
retarded when the plants were treated
with cool water (Sonku and Sakurai,
1973). Such effects may be associated
with the effect of temperature on the
transmission of virus to rice plants by the
insect vector.


Vol. 11







Transmission of Rice Tungro Virus


Insects (no.)


if-


200


2 4 6 8 10 12 14 16 18 20 22


Days after acquisition feeding

Fig. 2. Retention of infectivity of Nephotettix virescens transmitting the
rice tungro virus at two different temperatures after an acquisition
feeding on diseased plants.


I Noninfective

SInfective



320C


I I I I


Jan. & June, 1975








Philippine Phytopathology


Infective insects(%)
00oo



80-



60



40


Vol. 11

70C //26-31 C


Consecutive days after acquisition feeding


Fig. 3. Daily percentage of tungro-infective adults of Nephotettix virescens
showing gradual loss of infectivity with time, difference in transmission


between the two temperature
of the insects at 70C.


Temperature has been reported to
affect virus transmission (acquisition and
inoculation) by insect vectors aphids
(Stegwee, 1960; Sylvester and Richard-
son, 1966; Rochow, 1967), and leafhop-
pers (Maramorosch, 1950 Sinha, 1967).
The effects of temperature on the trans-
mission of rice virus and virus-like diseas-
es such as black-streaked dwarf (Ishii and
Yoshimura, 1973), dwarf (Ishii, Yasuo,
and Yamaguchi, 1970), stripe (Yasuo,
Ishii, and Yamaguchi. 1965) and yellow
dwarf (Iwahashi, Nagai, and Goto, 1964:
Nagai, Iwahashi, and Goto, 1964; Ishii,
Yasuo, and Ono, 1969) by their insect
vectors have also been investigated. The
results of the present study on the effect
of temperature on the transmission of the
rice tungro virus by N. virescens followed


regimes, and retention of infectivity


the general trend that transmission in-
creases to a maximum with increasing
temperature. However, the present results
indicate that low temperature affects the
acquisition of the virus by the insect
more than it affects the inoculation of
the virus into the rice plant by the insect
(Fig. 1). Similar results were reported by
Sylvester and Richardson (1966) for the
pea aphid, Acythosiphum pisum, that
transmits the pea enation mosaic virus.

Although temperature may affect the
spread of tungro disease to some extent
(Table 1), the temperature in most tropi-
cal regions may not drastically affect
transmission of the virus by the insect
vector under natural conditions when the
virus source is present in sufficient quan-








Transmission of Rice Tungro Virus


tity in the rice tield. In the tropics, the
range of temperatures during a rice grow-
ing season is comparatively narrow and
the minimum temperature is seldom dele-
teriously low. For instance, the minimum
and maximum temperatures in Los
Bafios Philippines during 25 years were
15.6 and 37.80C, respectively (University
of the Philippines, 1972). The present
results show no striking difference in
transmission in the acquisition feeding at
temperatures between 25 and 380C nor
in the inoculation feeding at temperature
between 20 and 380C (Fig. 1).

Temperature affects the infective capa-
city of N, virescens in transmitting rice
tungro virus. When the temperature
ranged between 13 and.340C, the higher
the temperature, the higher was the infec-
tive capacity. Based on the data obtained
from serial transmission at 30-minute
intervals for 10 hours (Table 2), an infec-
tive insect can infect, at the maximum,
about 40 seedlings/day at 340C, whereas
it can infect only 31 seedlings/day at
130C Both values are greater than the 30
infected seedlings/infective insect/day
previously reported (Ling, 1974). Possi-
bly, a more capable infective insect in-
fects more seedlings under more favorable
conditions. At present it is not clear
whether an infective insect can infect
more rice seedlings during its entire life


at a high temperature or at a low one be-
cause the insect has a higher infective
capacity at a higher temperature (Table
2), but a shorter life span (Table 3).

Temperature affects the retention of
plant virus by its insect vector. Aphid-
borne plant viruses are often retained
longer by the insect vector at lower tem-
peratures (Kassanis, 1941; Miller, 1952;
Bradley, 1954; Sylvester, 1954; Heinze,
1959; Cockbain, Gibbs, and Heathcote,
1963; Sylvester and Richardson, 1966).
Leafhopper-transiitted plant viruses are
mostly persistent. That limits the study
of the effect of temperature on the re-
tention period of the tungro virus by N.
virescens to be longer at 130C than at
320C.

The results obtained from maintaining
viruliferous insects at 70C (Fig. 3) showed
at that temperature the infectivity did
not increase, nor did the virus become
persistent in the vector but the rate of
loss of infectivity was low, suggesting that
the retention period may be longer at low
temperatures. The longest retention
period found at 130C was 22 days after
acquisition feeding (Fig. 2). The retention
period could be longer when a large
sample of viruliferous insects were tested
under a low temperature, which is more
favorable for the retention of the virus.


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Transmission of Rice Tungro Virus


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YASUO, S., M. ISHII, and T. YAMAGUCHI. 1965. Studies on rice stripe disease. (1)
Epidemiological and ecological studies on the rice stripe disease in Kanto-Tosan
District in Central Part of Japan (in Japanese, English summary). J. Centr. Agr.
Exp. Sta. 8:17-108.


























I


Jan. & June, 1975







Phil. Phytopathol. 11:58-61
Jan. & June, 1975

RESPONSE OF THE ETIOLOGIC AGENT OF CITRUS GREENING
DISEASE IN THE PHILIPPINES TO TREATMENT
WITH BROAD SPECTRUM ANTIBIOTICS


A. L. MARTINEZ
Supervising Plant Pathologist
Bureau of Plant Industry, Lipa City


ABSTRACT

The etiologic agent of citrus greening disease in the Philippines was inactivated by
treatment with 1,000 ppm concentration of the broad spectrum antibiotics like achromy-
cin, bonnacycline, tetrachel, isphamycin, pharcycline, terramycin, septamycin, chloram-.
phenicol and technical tetracycline. These results strongly support the earlier reports that
the pathogen is not a virus but a mycoplasma-like organism.


Greening is a disease of citrus known
locally in the Philippines as yellows,
leaf mottling and leaf-mottle-yellows
(Martinez and Wallace, 1967a; 1967b;
1968; Salibe and Cortez, 1968). This
disease has been responsible for the
decline and death of more than a mil-
lion citrus trees in this country since
1957 (Martinez et al. 1971; Martinez
and Wallace, 1969). In earlier studies
(Martinez and Wallace, 1967a; 1967b;
1968; Salibe and Cortez, 1968), a virus
has been assumed as the etiologic agent
of the Philippine greening disease. This
assumption is based mainly on the trans-
missibility of greening by tissue grafting
and by the psyllid, Diaphorina citri
Kuway. Later studies (Martinez et al.
1970; 1971; Martinez et al. 1971), how-
ever, have shown that symptoms of green-
ing are suppressed by treatment with
some tetracycline antibiotics which
suggests that a mycoplasma-like organism
is involved rather than a virus as origi-
nally assumed.
This paper presents the results of more
recent investigations strongly supporting
the mycoplasma-like etiology of citrus
greening disease in the Philippines.


MATERIALS AND METHODS

Effect of antibiotics on the etiologic
agent of greening in infected budsticks
and on citrus viruses. Broad specturm
antibiotics namely achromycin (tetra-
cycline HC1), bonnacycline (tetracycline
HC1), tetrachel tetracyclinee HC1) ispha-
mycin chlortetracyclinee HC1), pharcy-
cline oxytetracyclinee HCI), terramycin
oxytetracyclinee HC1) septamycin (7-
chlorotetracycline HCI), a pure form of
tetracycline, chloramphenicol and penta-
cillin (penicillin 500) were used in 5 sepa-
rate trails. Greenhouse-grown citrus plants
experimentally infected with greening by
D. citri were maintained as sources of
inocula. I lealtly test seedlings of various
citrus cultivars grown from seeds in the
greenhouse were bud-inoculated (3 buds
in each test seedling) from budsticks
of the greening-infected plant sources
previously immersed for 25 minutes in
1,000 ppm solution of each of the dif-
ferent antibiotics. A number of healthy
test seedlings were also bud-inoculated
by means of untreated buds from the
same infected plant sources. Non-ino-








Antibiotic Treatment of Citrus Greening


culated healthy seedlings were provi-
ded as controls. The same antibiotics
above were tested in the same way in
3 trials on the viruses of tristeza, seedling-
yellows, exocortis and psorosis.
Effect of antibiotics on the etiologic
agent of greening in infected plants. -
Three groups of test plants consisting
of potted seedlings and budlings of va-
rious citrus cultivars showing severe
greening symptoms after experimental
infection by D. citri were used in this
study. The test plants of the first group
were treated by spraying (to run off)
all above-soil parts with a 100 ppm so-
lution each of achromycin, tetrachel,
isphamycin, pharcycline, terramycin and
chloramphenicol. Spray treatment was
administered once daily for a period of
12 weeks. The test plants of the second
group were placed singly in separate
containers with 100 ppm solution each
of the 6 antibiotics and left there for
12 weeks. The test plants of the third
group were maintained as untreated
controls.
Reaction of healthy plants pre-treated
with antibiotics to the greening. -
Potted healthy seedlings of various
citrus cultivars were treated separately
by foliar spray (to run off) with 100
ppm solution each of achromycin, te-
trachel, isphamycin and terramycin.
Foliar spraying was done once a day
for a period of 4 weeks. Simultaneously,
other potted healthy seedlings of the same
citrus cultivars were treated by directly
placing each seedling in containers with
the same concentration of the 4 anti-
biotics and left there for 4 weeks. lalf
of the seedlings of each treatment was
tissue-graft inoculated from a psyllid-
transmitted greening source maintained
in citrus plants. The remaining half in
each treatment was retained as controls.
Non-antibiotic-treated healthy seedlings
were also tissue-graft inoculated from the


same greening-infected plant source.
Non-inoculated, non-treated healthy seed-
lings were provided as additional controls.

RESULTS
All test seedlings inoculated with
untreated infected buds and all test
seedlings inoculated with pentacillin-
immersed infected buds developed typical
greening symptoms within 2-3 months.
None of the test seedlings inoculated
with infected buds previously immersed
in 1,000 ppm solutions of achromycin,
bonnacycline, tetrachel, isphamycin,
pharcycline, terramycin, septamycin, a
pure form of tetracycline and chloram-
phenicol developed symptoms within 16
months (Fig. 1). Their foliage was as
normal as that of the non-inoculated
controls. Additionally, they produced
many symptombless auxiliary and/or
secondary shoots. The 10 antibiotics
:*lop.,

IR ^


Fig. 1. Sziwuikom mandarin seedlings.
Non-inoculated healthy control,
left; inoculated with antibiotic
treated infected buds, center;
inoculated with non-treated in-
fected buds, right. Photo taken
6 months after inoculation.


Jan. & June, 1975









Philippine Phytopathology


had no ettect on the transmission of or
symptoms induced by the 4 citrus viruses.
A majority of the antibiotic-treated
test plants of the first and second groups
showed partial recovery; none were
completely cured. They developed more
chlorophyll and produced more symp-
tomless side and terminal shoots than the
controls (Fig. 2). Most of these symp-
tomless shoots, however, eventually
showed greening symptoms when treat-
ment was discontinued.


Fig. 2. Sampson tangelo seedlings. Left,
non-treated severely infected
control, and right, antibiotic -
treated severely infected test
seedling. Note a symptomless
shoot produced on the treated
plant. Photograph taken 10
weeks after treatment.
When some of the antibiotic-treated
test plants in both groups were separate-
ly used as sources of bud-inoculum,
12 per cent of the bud-inoculated test
seedlings developed greening symptoms.
But when the non-treated plants were
used as bud-inoculum sources, 100o
of the bud-inoculated test seedlings de-
veloped greening symptoms.


Within 2-3 months, all the tissue-
graft inoculated, non-antibiotic-treated
seedlings developed typical greening
symptoms. None of the tissue-graft
inoculated, antibiotic-treated seedlings in
the two treatments developed symp-
toms within 10 months. They did not
differ from the non-inoculated, non-
treated controls.


DISCUSSION

In the present investigations, the ab-
sence of typical symptoms in test seed-
lings inoculated with antibiotic-treated,
greening-infected buds, the failure of
antibiotics to prevent transmission of the
viruses of tristeza, seedling-yellows, exo-
cortis and psorosis, the partial recovery
of severely infected plants treated with
antibiotics, and the non-development of
symptoms in healthy test seedlings pre-
treated with antibiotics amply demons-
trate successful inactivation of the
etiologic agent of citrus greening di-
sease by broad spectrum antibiotics.
These results strongly support the ear-
lier reports (Martinez et al. 1970; 1971;
Martinez et al. 1971) that the etiologic
agent of citrus greening disease in the
Philippines is a mycoplasmalike orga-
nism and not a virus.
Lafleche and Bove (1970a; 1970b)
reported the presence of mycoplasma-
type structures in the sieve tubes of
greening-affected citrus plants. In their
later studies (Lafleche and Bove, 1972),
they found that the envelope of the
structures associated with greening
disease was 200. A thick and concluded
that it could not consist of only a unit
membrane but probably has a wall asso-
ciated with it. They stated that, the
greening structures cannot be classified
within the Mycoplasmatales and sug-
gested that they could be a new type of


Vol. 11









Antibiotic Treatment of Citrus Greening


phytopathogen. The thick envelope was structures associated with the afore-
observed on structures found in plants mentioned group of citrus diseases,
infected with South African greening, they appear to react like true mycoplas-
Reunion greening, India citrus decline, ma to treatment with antibiotics. The
and Philippine leaf mottling (greening) final proof of mycoplasmalike etiology
disease (Lafleche and Bove, 1972). of this group of diseases awaits the
complete fulfillment of the Koch' nne-
Regardless of the true nature of the tulates of pathogenicity.




LITERATURE CITED

LAFLECHE, D. and J. M. BOVE. 1970a. Cytopathologie vegetable Structures de type
mycoplasme dans les feuilles d'orangers atteints de la maladie du 'greening'. C. R.
Acad. Sci., Paris 270: 1915-1917.
LAFLECHE, D. and J. M. BOVE. 1970b. Mycoplasmes dans les agrumes atteints de
"greening," de "stubborn" ou de maladies similaires. Fruits 25:455-465.
LAFLECHE, D. and J. M. BOVE. 1972. Microorganisms associated with greening and
similar disease of citrus. Proc. 6th Conf. Intern. Organization Citrus Virol. (In
Press).
MARTINEZ, A. L., D. M. NORA and A. L. ARMEDILLA. 1970. Suppression of symp-
toms of citrus greening disease in the Philippines by treatment with tetracycline
antibiotics. Plant Dis. Reptr. 54: 1007-1009.
MARTINEZ, A. L., D. M. NORA and A. L. ARMEDILLA. 1971. Effects of antibiotics on
citrus greening pathogen. Phil. Phytopathol. 7: 5-6.
MARTINEZ, A. L., D. M. NORA and W. C. PRICE. 1971 Observations of greening in the
Philippines. Animal Husbandry and Agric. Jour. 6: 21-22.
MARTINEZ, A. L. and J. M. WALLACE. 1967a. A progress report of studies on citrus
decline in the Philippines. Phil. Jour. Plant Ind. 32: 253-262.
MARTINEZ, A. L. and J. M. WALLACE. 1967b. Citrus leaf-mottle-yellows disease in the
Philippines and transmission of the causal virus by a psyllid, Diaphorina citri. Plant
Dis. Reptr. 51: 692-695.
MARTINEZ, A. L. and J. M. WALLACE. 1968. Studies on leaf-mottle-yellows disease of
citrus in the Philippines. pp. 167-176. In J. F. L. Childs (ed.), Proc. 4th Conf.
Intern. Organization Citrus Virol. Univ. Florida Press, Gainesville.
MARTINEZ, A. L. and J. M. WALLACE. 1969. Citrus greening disease in the Philippines.
In H. D. Chapman (ed.), Proc. First International Citrus Symposium 3: 1427-1431.
SALIBE, A. A. and R. E. CORTEZ. 1968. Leaf mottling a serious disease of citrus in
the Philippines. pp. 131-136. In J. F. Childs (ed.), Proc. 4th Conf. Intern. Organiza-
tion Citrus Virol. Univ. Florida Press, Gainesville.


Jan. & June, 1975







Phil. Phytopathol. 11:62-71
Jan. & June, 1975

GREENHOUSE AND FIELD TESTS OF PROTECTIVE FUNGICIDES
FOR THE CONTROL OF COFFEE RUST

R.B. VALDEZ, J.R. ACEDO, and C.O. DIROZO

Associate Professor and former Research Assistants, respectively, Department of
Plant Pathology, College of Agriculture, University of the Philippines at Los Bafos.
This research was supported in part by a grant from the National Science Develop-
ment Board (Project No. 2.53.1).

ABSTRACT

A series of greenhouse and field spray experiments were conducted to evaluate
the efficacy of new and commercial fungicides for the control of coffee rust, Hemileia
vastatrix Berk. and Br.
Locally available proprietary fixed coppers in the form of oxides, oxychlorides and
sulfates and some carbamates, e.g., ziram, ferbam and zineb were found to be equally
effective as the standard 3-3-50 home-made Bordeaux mixture. Two tin compounds were
also found to be promising. Captan, Maneb, Glyodin, Cosan, Vancide M and Antracol
were ineffective.
Two types of phytotoxicity, viz., the chlorotic type typified by Melprex and the
necrotic lesion type produced by the copper- and the zinc-based compounds were ob-
served. The chlorotic type predisposed the sprayed trees to sun-scorching during summer
while the necrotic lesion type varied in the degree of severity and in the size and color
of the lesions between and within the copper and the zinc-based compounds. However,
their possible detrimental effects on the yield of sprayed coffee trees are still unknown.


Coffee rust, caused by Hemileia
vastatrix Berk. and Br., has been present
in the Philippines since the last decade
of the 19th century. It was reported to
have caused the downfall of the coffee
industry that once ranked fourth as an
export trade of the Philippines in 1890
and has caused the main coffee growing
area to shift to citrus.

The use of protective fungicides,
notably Bordeaux mixture as well as the
proprietary coppers and other organic
compounds including antibiotics applied
either alone or in combination with
stickers to control rust on Arabica coffee,
has been investigated by several workers
(Dowson, 1921; Mayne, 1940; Thomas,
1945, 1950; George, 1956-1963; Rayner,
1962; Bock, 1962; Wallis and Firman,
1962; Burdekin, 1962, 1964; Park and
Burdekin,' 1964; Nutman, 1963; Walle,
1961; Wellman, 1955, 1959, 1961; Fir-


man and Wallis, 1965). In the Philippines,
the use of Bordeaux mixture to control
this disease was first demonstrated by
Luistro (1915) and later its effective
minimum spray application was shown by
Africa (1918) to be 2-1/4-3-2/3-50
formula put on at 2-week intervals.
Newhall and Orillo (1954) reported that
Yellow cuprocide was comparable in
effectiveness as a 5-5-50 Bordeaux mix-
ture. In a field evaluation of fungicide-
sticker combinations against coffee rust,
Valdez et al. (1959) showed that Parzate
plus Goodrite p.e.p.s. gave the least per-
centage of rusted leaves. Although Bor-
deaux mixture has been and is still re-
commended as a standard fungicide in
the Philippines, more recently because of
the advent of proprietary products that
are more convenient to apply, it has not
been widely adopted by most growers
because of the difficulty and length of its
preparation, incompatibility with other








Fungicides for Coffee Rust


pesticides, and necessity to apply it soon
after preparation.

The present paper reports the results
obtained in assessing the protective
properties of new fungicides under green-
house conditions and the effect of selec-
ted materials as compared with commer-
cially available' compounds under field
conditions against coffee rust. The experi-
ments were conducted from 1962-67
in the greenhouse and in the orchard of
the Department of Plant Pathology,
College of Agriculture, University of the
Philippines at Los Bafios, Laguna.


MATERIALS AND METHODS

Screening tests

Greenhouse screening. Coffee arabica
L. typical seedlings, obtained from a
known rust-susceptible tree, were grown
in five greenhouse beds, 3.5 ft. x 9.5 ft.
each. Twelve rows were planted in each
bed with nine seedlings to a row. The
treatmerits were randomized in each of
the five beds with one row representing
one treatment and one bed representing
a replication. The same plants and the
same layout were used during the follow-
ing year, but since the plants have already
grown too fall, they were pruned to
about six inches about 5 months before
coffee rust spores usually become avail-
able. Two to three shoots were allowed
to grow per plant and during this period
about three pairs of leaves were ready
for inoculation.
Field screening. In view of the poor
results obtained during the first two
years using greenhouse-grown plants, the
subsequent screening tests were conduc-
ted on bearing rust-susceptible field-
grown Arabica trees. The layout consisted
of having all the treatments including the


check distributed at random on well
spaced marked branches of one tree with
one pair of half-developed leaves in each
branch representing a treatment. At least
three trials were usually made during one
rust season From September to December
using 15 single-tree replications.

Treatments. The fungicides tested
and screened for effectiveness against
coffee rust in both greenhouse and
field tests were Wepsyn WP.155 (25%
5-amino-l-bis(dimethyl amide) phos-
phoryl-3-phenyltriazole-l,2.4); Vancide A
ME 4698 (experimental); Cosan (80% col-
loidal sulfur); Actispray (7.7% cyclohexi-
mide [B-(2-(3,5-dimethyl-2-oxocyclohe-
xyl)-2-hydroxyethyl)-glutarimide); Antra-
col (70% zinc ethylene bisdithiocarba-
mate); Caocobre (50% cuprous oxide);
Vancide M (30% manganesedimethyl
dithiocarbamate + 2-mercaptobenzothia-
zole); Miller Fungicide 658 (95% copper
zinc chromate complex); Difolatan (80%
N-(1,1,2,2-tetrachloroethylthio)-4-cyclo-
hexene-l,2 dicarboximide); 0-3818 B
(nickel salts of zineb analog); 0-3818 M
(nickel salts of maneb analog); Brestan
60 (60% triphenyltinacetate); Dithane
M-22 (80% manganese ethylene bisdi-
thiocarbamate); Glyodin (30% 2-hepta-
decyl glyoxalidine acetate); Melprex
(65% triphenyltin hydroxide). Brestan,
Melprex, Du-ter and Antracol were
applied at the rate of-1 lb/100 gallons
each; Wepsyn WP 155, Vancide M,
Vancide A ME 4698, Copper lonacol,
Miller Fungicide 658, Difolatan, 0-3818
B, 0-3818 M and Dithane M-22 at 2 lbs./
100 gallons, Cosan at 3 lbs./100 gallons;
Caocobre at 4 lbs./ 100 gallons; Agrimy-
cin 500 at 5 lbs/100 gallons; Glyodin
at 2 pints/100 gallons; and Actispray at
500 ppm. A 3-3-50 home-made Bordeaux
mixture prepared immediately before
application served as the standard of com-
parison. Where there was no information


Jan. & June, 1975








Philippine Phytopathology


that the fungicide was previously tested
against rust or other coffee diseases,
the range of safety on coffee was first
determined by applying different dosages.
The dosage rate tolerated by the plant
was then evaluated for its toxicity to the
rust fungus. About 10-12 test fungicides
were evaluated during each yearly trial.
The pathogen-chemical-suscept method
was used in the screening tests. Suspen-
sion of each fungicide was prepared
separately in Erlenmeyer flasks using
distilled water and was sprayed on the
nether surface of selected half-developed
leaves. The sprayed leaves were first
allowed to dry thoroughly and later
inoculated with a suspension of rust
spores. De Vilbiss atomizers No. 15
were used in spraying both fungicides and
'rust spores. In the greenhouse, bamboo
troughs filled with water were placed
between the rows, the soil bed fully
saturated with water and after inocula-
tion the entire bed was covered immed-
iately with black polyethylene sheets
mounted on wooden supports that served
as improvised moist chamber during the
incubation period of from 48-72 hours.
In the field, no improvised moist cham-
bers were provided. In both greenhouse
and field screening tests, the fungicides
were evaluated 6 weeks after inoculation
by counting the number of rusted leaves,
the number of rust lesions per leaf and
also by noting down the degree of sporu-
lation after every successful inoculation
trial. The phytotoxic effects and symp-
toms of the different test chemicals on
the leaves were likewise observed and
noted throughout the duration of the
experiment. In this way, only the promis-
ing materials were tested under field con-
ditions.

Field tests
Different fungicides were evaluated
under field conditions on 8-10 years old


rust-susceptible Arabica trees grown at
160 ft above sea level. All the yearly trials
were set in randomized block design with
each treatment, including the check,
replicated four times with five trees per
replication. Guard or alternate trees were
provided between treatments and left
unsprayed to minimize contamination by
spray drifts and to serve as source of ino-
culum.
Treatments. The fungicides that were
evaluated were not only those commer-
cially available but also those found pro-
mising in the screening tests. They were
Duphar Colloidal copper (27% metallic
copper); Perenox (50% cuprous oxide);
Cupravit Ob-21 (84% copper oxychloride);
Copper A Compound (45% metallic cop-
per); Shell Copper Fungicide (86% copper
oxychloride); C-O-C-S (26.5% metallic
copper); Spraycop 530 (53% metallic
copper); Acme Bordeaux (12.75% metal-
lic copper); Cuprox(87% copper oxychlo-
ride); Caocobre (50% cuprous oxide)
Copper Lonacol (25% copper oxychlo-
ride + 15% cuprous oxide); Copper
Lonacol (25% copper oxychloride +
15% zinc. ethylenebisdithiocarbamate);
Miller Fungicide 658 (95% copper zinc
chromate complex); Cosan (80% colloi-
dal sulfur); Fermate (76% ferric dime-
thyldithiocarbamate); Parzate C (75%
zinc ethylenebisdithiocarbamate); Ziram
(76% zinc dimethyldithiocarbamate);
Manzate (70% manganese ethylenebis-
dithiocarbamate); Antracol (70% zinc
thiocarbamate); Orthocide 50 (50% N-
trichloromethyl mercapto-4-cyclohexene-
1,2,-dicarbomixide); Phygon-XL (50%2,3-
dichloro-1, 4-napthoquinone); Vancide M
(30%manganese dimethyldithiocarbamate
+ 2-mercaptobenzothiazole); Brestan 60
(60%triphenyltin acetate); Melprex (65%
dodine (N-dodecylguanadine acetate);
Du-Ter WP (50%triphenyltin hydroxide);
and Glyodin (30% 2-heptadecyl glyoxali-
dine acetate) The rates of application


Vol. 11








Fungicides for Coffee Rust


used per 100 gallons of water were 1
lb each for Phygon-XL Brestan 60,
Antracol, Melprex and Du-Ter; 1.5 lbs
for Miller Fungicide 658; 2 lbs each for
Fermate, Parzate C, Ziram, Manzate,
Cosan, Vancide M, and Copper Lonacol;
2.5 lbs for Orthocide 50; 3 lbs each for
Perenox, Cupravit, Copper A Compund,
Shell Copper Fungicide, C-O-C-S, and
Spraycop 530; 4 lbs each for Caocobre
and Cuprox, 5 lbs for Acme Bordeaux;
2 pints for Glyodin and 4 pints for Colloi-
dal Copper. The dosage rates adopted for
Cosan and all the copper compounds
were those recommended by the manu-
facturers as indicated in the product's
label, while those of the rest were either
based from the findings in the greenhouse
tests or from the rates used by other
workers. A 3-3-50 home-made Bordeaux
mixture, prepared immediately before
use, served as the standard for compari-
son. All the treatments were applied at
3-weekly intervals by means of pneumatic
knapsack sprayers at the rate of 500
gallons per hectare or approximately
three pints per tree.

Evaluation of the protective ability
of the test fungicides was made by count-
ing 300 leaves taken at random from all
positions, i.e. from N. S. E. W. quadrants
of the low, middle and top portions of
each of the 20 single tree-replications in
each treatment four weeks after the last
spray application and indicating the num-
ber of healthy, rusted as well as those
defoliated due to rust. During the sam-
pling, care was taken such that the leaf
counts were made only from the growth
increments within the duration of the
spray applications. For instance, it has
been determined by Valdez (unpublished)
that a branch of a bearing tree would put
on from 4-5 pairs of leaves during the
spraying period from May to December
and that 74 per cent of the leaves formed
in one year were produced during the


months of May and June. Hence, leaf
counts were taken from within this range.
In this way, leaves that were already
infected before the first spray was applied
were not included in the counts.

The phytotoxic effects of the test
fungicides manifested on the leaves
were evaluated soon after the rust read-
ing was completed. This effect was as-
sessed by getting 15 leaf samples at ran-
dom from each of the low, middle and
top portions of each tree-replication.
From these, the percentage of leaves
showing toxicity lesions and the density
of lesions per leaf were determined. The
range of sizes of the lesions was obtained
by measuring the diameter of 200 lesions
of varying sizes obtained from a mini-
mum of 20 leaves in each treatment.

The monthly climatological conditions,
namely, temperature, rainfall, relative
humidity and number of rainy days,
recorded during the period of this experi-
ment in the U.P. College of Agriculture
weather station which was located about
a kilometer away from the experimental
orchard was obtained.

RESULTS AND DISCUSSION

Phytotoxicity of fungicides

Screening Test. Among the 18 fungi-
cides screened for effectiveness against
coffee rust, Actispray, Melprex, Miller
Fungicide 658,0-3818 B, 0-3818 M, Bres-
tan 60, Du-Ter WP, and Glyodin exhibi-
ted phytotoxicity on coffee in varying
degrees. Although none of these have
caused death of the plants, Actispray
applied at 50 ppm and Melprex at 1 lb/
100 gallons appeared to be the most toxic.
Phytotoxicity of Melprex (Cyprex) has
been reported by George (1960) to.have
caused premature leaf fall which was no-
ticed 2-3 weeks after the first application.


Jan. & June, 1975








Philippine Phytopathology


George (1961) also reported that 0-3818
B exhibited scorching of leaves 3 weeks
after the first round of tertiary branches.
Except Miller Fungicide 658, the phyto-
toxicity symptoms produced by the
above-mentioned fungicides were of the
chlorotic type. Immediately after applica-
tion, watersoaked brownish circular pat-
ches appeared on the neither surface of
the treated leaves and after 2-3 weeks
these affected areas became chlorotic.
However, when only a portion of the
plant has been sprayed, such as a pair
of leaves, the symptoms gradually dis-
appeared and the affected leaves later.
became green.

Field Test. The results of the field
tests showed two types of phytotoxicity
manifested on the leaves, namely, chlo-
rotic type and minute necrotic lesion
type. The symptoms of the former were


gradual chlorosis of both young and old
leaves. This type of phytotoxicity was
prominent on trees sprayed with Mel-
prex and slightly on those treated with
Cosan. This effect rendered the sprayed
trees more susceptible to sun-scorching.
On the other hand, the symptoms of the
latter (Fig. 1) start as pinhead-size nec-
rotic lesions which were visible on both
leaf surfaces and surrounded by a yellow
halo when viewed against the light. The
lesions, depending upon the spray mater-
ial used, slowly increased in size as the
sprayed leaves grew older and as repeated
applications were put on. The trees
sprayed with Ziram gave the highest
percentage (81%) of leaves with minute
necrotic lesions, Table 1. This was fol-
lowed by Perenox, Caocobre, Duphar
Colloidal Copper, Miller Fungicide 658,
Parzate and Bordeaux mixture. Each
of these caused toxicity lesions in more


Fig. 1. Leaves of Arabica coffee showing necrotic spots caused by fungicide
toxicity (1) Ziram, (2) Parzate, (3) Duphar Colloidal Copper, and (4)
Perenox.


Vol. 11








Fungicides for Coffee Rust


Table 1. Effects of fungicide toxicity on the number and size of necrotic lesions on leaves
of sprayed A rabica coffee1


Per cent Mean number Size range of
T r e a t m e n t leaves w/ of lesions lesions
lesions per leaf (mm)

Miller Fungicide 658 69.2 27.18 0.25-2.50
Duphar Colloidal Copper 70.7 24.03 0.25-1.75
Ziram 81.0 21.83 0.25-2.50
Parzate 66.5 19.43 0.25-3.75
Bordeaux mixture 56.5 10.97 0.25-2.00
Perenox 77.4 10.63 0.25-2.00
Caocobre 73.6 10.10 0.25-2.25
Cupravit Ob-21 42.8 7.90 0.25-2.00
Cuprox 40.0 7.00 0.25-2.00
Spraycop 530 43.2 6.80 0.25-2.00
Antracol 45.5 5.70 0.25-2.50
C-O-C-S 39.0 5.61 0.25-2.50
Copper Lonacol 42.3 3.85 0.25-1.50
Acme Bordeaux 36.2 2.00 0.25-1.50

1Based on 45 leaf samples per tree with 20 trees per treatment.


than 50 per cent of the leaves with each
leaf having an average of more than ten
lesions. On the other hand, those sprayed
with Acme Bordeaux gave the least per-
centage of leaves with lesions, the lowest
number of lesions per leaf as well as one
of the smallest sizes of lesions. Among
the fungicides tested, only those in which
the active ingredients were either zinc or
copper or both exhibited this type of
toxicity symptoms. The sizes of lesions
produced by the zinc-based fungicides
ranged from 0.25-3.75 mm with the ma-
jority falling in the 1-2 mm groups, while
those produced by the copper-based
materials ranged from 0.25-1.5 mm
groups.

Similar types of phytotoxicity of
copper fungicides were also reported on
Dioscorea alata (Singh and Prasad, 1966),
tomato (Horsfall et al. 1937), and on
peach, apples and lettuce (McCallan and
Wilcoxon, 1938).


Effect of fungicides on rust control;

Greenhouse-and Field-screening tests.
Results of screening fungicides using
Arabica seedlings grown in greenhouse
beds were unsatisfactory in spite of
efforts to adjust the conditions favorable
for infection and development of the
rust fungus. However, the use of field-
grown susceptible but rust-free Arabica
trees was found to be very satisfactory
and practical. Elimination of ineffective
fungicides was based on consistent results
obtained under field conditions.
Field Test. Results of five yearly
spraying experiments conducted under
field conditions comparing 26 proprietary
formulations of coppers, carbamates, tins,
and other compounds with home-made
Bordeaux mixture as standard of com-
parison are summarized in Table 2. The
protective effects of some of these ma-
terials on sprayed Arabica trees are shown
in Fig. 2a-d.


Jan. & June, 1975.









Philippine Phytopathology


Table 2. Efficacy of 26 protectant fungicides in the control of leaf rust of Arabica
coffee'



Percentage of
Treatment2
Rusted leaves3 Disease control4

Duphar Colloidal Copperb 5.15 92.6
Cuproxe 6.30 90.9
Caocobree 7.12 89.8
Bordeaux mixture 7.24 89.6
Cupravit Ob-21b 7.29 89.5
Acme Bordeauxc 7.82 88.7
Perenoxb 7.88 88.7
Shell Copper Fungicideb 8.77 87.4
Copper Lonacold 9.59 86.2
Brestan 60d 9.61 86.2
Copper A Compoundd 11.13 84.0
Ziramb 11.22 83.9
Miller Fungicide 658b 11.59 83.3
Fermateb 12.02 82.7
Spraycop 530e 12.35 82.2
C-O-C-Sd 13.67 80.3
Parzate Cb 15.41 77.8
Du-Tere 20.66 70.3
Melprexe 20.71 70.2
Phygon-XLe 21.35 68.9
Orthocide 50b 22.57 67.5
Vancide Me 22.63 67.4
Cosanc 28.04 59.6
Manzateb 29.42 57.7
Antracold 29.43 57.7
Glyodine 42.80 38.3
Control 69.50
H.S.D. .05 14.77
.01 13.07

1 Based on 6,000 leaves per treatment of 20 trees each with 300 leaf samples per tree in
any one of 5-year trials.
2 Number of times a fungicide was tested: a = 5 X, b = 4 X, c = 3 X, d = 2 X, e = 1 X.
3 Mean arcsine percentage of five yearly trials; means for treatments with less than five
trials were calculated based on performance of Bordeaux mixture.
4 Calculated by the following formula:
% rusted leaves % rusted leaves
% disease in control in treatment
SX 100
control % rusted leaves in control


Vol. 11








Jan. & June, 1975


Fungicides for Coffee Rust
ANWr W11rF


Fig. 2a-d. Arabica coffee showing the effect of fungicidal sprays for the
control of rust; (a) Bordeaux Mixture, (b) Duphar Colloidal
Copper, (c) Manzate, and (d)


Table 2 shows that all the treatments
gave highly significant differences over
the untreated check. However, there
were no significant differences in terms
of rusted leaves in both 1% and 5% le-
vels between any of the following fungi-
cides and the standard: Duphar Colloi-
dal Copper, Cuprox, Caocobre, Cupra-
vit, Acme, Bordeaux, Perenox, Shell


Copper Fungicide, Copper Lonacol, Bres-
tan 60, Fermate, Spraycop 530, C-O-C-S
and Parzate C. These results corroboated
the findings of Burdekin (1964) when he
reported that zinc fungicides gave equally
good control of leaf rust as the copper
fungicides.

Du-Ter, Melprex and Phygon-XL gave








Philippine Phytopathology


vel. On the other hand, Orthocide 50,
Vancide M, Cosan, Manzate, Antracol
and Glyodin showed highly significant
differences when compared with the
standard and have been found to be
consistently ineffective. The-same unsatis-
factory results with Orthocide (Captan)
was obtained by Burdekin (1961) in his
work in Tanganyika. The findings of
George (1957, 1959) also corroborated
the above results when he reported that
control with ultra-sulfur gave the poorest
leaf retention.

During the five-year period of this
experiment, the climatological conditions
recorded during the months of May whe
the coffee rust fungus starts to build up
until January when practically all the
infected leaves are shed, showed that
significant differences only at the 1% le-


the temperature, relative humidity, rain-
fall and number of rainy days and a mean
of 26.oC, 86.1% inches and 19.9 days,
respectively.

Indeed, there are now several subs-
titutes for Bordeaux mixture which are
cheap and yet equally effective. Among
these possible Bordeaux substitutes, the
fixed coppers in the form of oxides,
oxychlorides and sulfates were evidently
superior over the carbamates, viz., ziram,
ferbam and zineb. From repeated trials,
Cosan, Captan, Maneb, Glyodin, Antracol
and Vancide M proved to be ineffective
against coffee rust.

Nevertheless, in the choice of a fungi-
cide one should consider not only effi-
cacy but also availability and economy.


LITERATURE CITED

AFRICA, E. M. 1918. The minimum Bordeaux application for the control of Hemileia.
The Philippine Agriculturist and Forester 6: 251-270.
BOCK, K. R. 1962. Control of coffee leaf rust in Kenya colony. Trans Brit. Mycol. Soc.
45: 301-313.
BURDEKIN, D. A. 1961. The effect of captain and copper sprays on leaf rust and leaf
leaf fall of coffee. Res. Rept. Coffee Res. Station Lyamungu, Tanganyika, 1960.
56-59.
_. 1964. The effect of various fungicides on leaf rust, leaf retention
and yield of coffee. East Afric. Agric. and For. Jour. 30 (2): 101-104.
DOWSON, W. J. 1921. Some problems of economic biology in East Africa (Kenya Co-
lony). Ann. Appl. Biol. 8 (2): 83-100.
FIRMAN, I. D. and J. A. WALLIS. 1965. Low-volume spraying to control coffee leaf rust
in Kenya. Ann. Appl. Biol. 55: 123-137.
GEORGE, K. V. 1956. Eighth Ann. Report of the Res. Dept. of Indian Coffee Board,
1954-1955. pp. 77-85.
1957. Ninth Ann. Rept., 1955-1956. pp. 70-80.
1958. Tenth Ann. Rept., 1956-1957. pp. 88-104
1959. Eleventh Ann. Rept., 1957-1958. pp 89-103.
_. 1960.Twelfth Ann. Rept., 1958-1959. pp. 93-113.
1961.Thirteenth Ann. Rept., 1959-1960. pp. 165-177.
1962. Fourteenth Ann. Rept., 1960-1961. pp. 189-219.
1963. Fifteenth Ann. Rept., 1961-1962, pp. 44-48.
HORSFALL, J. G., R. O. MAGIE, and C. H. CUNNINGHAM. 1937. Effect of copper
sprays on ripening of tomatoes. Phytopathology 27: 132.
LUISTRO, F. D. 1915. Study of native coffee production. The Philippine Agriculturist
and Forester 4: 153-161.
MAYNE, W. W. 1940. Report on comparative trials with Perenox carried out at Balehon-
nur and Sidapur in 1939-1940. Planter's Chronicle 35: 345-347. (Abst. in Rev.
Appl. Mycol. 19: 700-701).


Vol. 11









Fungicides for Coffee Rust


McCALLAN, S. E. A., and F. WILCOXON. 1938. Laboratory comparison of copper
fungicides. Contrib. Boyce Thomp. Inst. 9: 249-263.
NEWHALL, A. G., and F. T. ORILLO. 1954. Coffee control found. U.P.C.A. Monthly
Bull. 19 (6): 1, 3.
NUTMAN, J. 1963. The control of diseases of coffee in Kenya. Indian Coffee 27 (5):
137-141.
PARK, P. O., and D. A. BURDEKIN. 1964. Studies on the application of a copper fungi-
cides for the control of coffee leaf rust. Ann. Appl. Biol. 53: 133-150.
RAYNER, W. D. 1962. The control of coffee rust in Kenya with fungicides. Ann. Appl.
Biol. 50: 245-261.
SINGH, R. D., and N. PRASAD. 1966. Efficacy of different fungicides for control of
anthracnose of Dioscorea alata. Plant Disease Reptr. 50 (6): 385-387.
THOMAS, K. M. 1949. First Ann. Rept. of the Res. Dept. of the Indian Coffee Board,
1947-1948. (Abst. in Rev. Appl. Mycol. 29: 210)
_. 1950. Second Ann. Rept., 1948-1949. (Abst. in Rev. Appl. Mycol.
30: 366)
VALDEZ, R. B., A. N. PORDESIMO, and F. T. ORILLO. 1959. A field evaluation of six
stickers in combination with Parzate for the control of coffee rust. Plant Disease
Reptr. 43 (5): 562-564.
WALLE, E. VAN DE. 1961. The coffee rust in Kivu, description and control. In Hofchen-
Briefe 14 (1): 15-30. (English ed.)
WALLIS, J. A. N. and I. D. FIRMAN. 1962. Spraying Arabica coffee for the control of
leaf rust. East Afric. Agric. and For. Jour. 28 (2): 89-104.
WELLMAN, F. L. 1955. Coffee diseases, insects and weeds controlled by chemicals. In
advances in Chemistry Series No. 13: Pesticides in tropical agriculture pp. 43-63.
1959. Recent spraying for control of coffee diseases. In Coffee an'd
Tea Industries and the Flavor Field. 81 (11): 106-111.
1961. Coffee: Botany, cultivation and utilization. Interscience Pub-
lishers, Inc. N.Y. xviii + 488 pp.


Jan. & June, 1975







Phil. Phytopathol. 11:72-79
Jan. & June, 1975



EFFECT OF DEEP PENETRANT AND OTHER
ADJUVANTS AS ADDITIVE TO
FUNGICIDES IN THE CONTROL
OF RICE BLAST

DELFIN B. LAPIS and NENITA L. OPINA

Assistant Professor and Research Assistant, respectively, Department of Plant
Pathology, College of Agriculture, UPLB, College, Laguna.
This research was supported by UPLB-CA-PCARR Joint.Project No. 20(4).

ABSTRACT

Laboratory bioassay, greenhouse and field experiments were conducted to deter-
mine the tenacity and efficacy of different protective and systemic fungicides as induced
by Deep Penetrant and other adjuvants in controlling rice blast.
Laboratory bioassays indicated that in all tests, Deep Penetrant did not give any
significant increase in the effectiveness of the fungicides. Tween 60 was proven to be
more effective than Deep Penetrant and Triton Act M.
Greenhouse and field experiments showed that the adjuvants insignificantly in-
creased the effectiveness of Benlate and Dithane M-45. Deep Penetrant was comparable
to other adjuvants.
Trial test in the farmer's field showed that the fungicide-adjuvant combinations
neither significantly controlled neck rot nor increased the yield of the rice crop.


Deep Penetrant (DP) is a new, revo-
lutionary and multi-purpose farm chemi-
cal claimed locally to be an effective
additive to fertilizers, insecticides, her-
bicides and fungicides applied as foliar
spray or soil drench. Among its impor-
tant properties are non-toxicity to plants
and animals, non-corrosiveness to farm
equipment, non-flamability and solubi-
lity with water, liquid fertilizers and
pesticidal solutions.
Reports indicated that DP induced
the uptake of fertilizers thereby in-
creasing the productive tillers and vigor
of plants resulting in increased rice yield
for about 23.1 to 103.7% (San Diego,
1970; and Donato, Manglicmut and
Gabucan, 1970). Alimagno and Ramos
(1971) claimed that addition of DP
significantly increased the uptake of
phosphorus in both corn and okra.
Several investigators reported that DP
is an effective additive and increased the.


efficacy of pesticidal solutions. Canoy
(1969) reported that DP when added
to weed killer (2, 4-D) gave a significant
increase in percent kill of the resistant
weed species from 51.4 to 93.2%
Schultz and Dalmacio (1970) found that
DP enhanced the effectiveness of chlo-
roneb (Demosan 65W) as foliar spray
against Sclerospora philippinensis Weston
but did not improve the effectiveness
of chloroneb when applied as seed
treatment. Quebral et al. (1970) showed
that incorporation of DP with fungi-
cides did not show any detectable effect
against Leveillula taurica (Lev.) Am.
This study was conducted to compare
the merit of DP with other adjuvant when
added to conventional fungicides in
controlling foliar blast of rice and to com-
pare the efficacy of DP with other ad-
juvants in enhancing the tenacity and
efficacy of protectant and systemic
fungicides.








Fungicide Adjuvants for Rice Blast


MATERIALS AND MI'TII1)SD

Isolation, Test of Pathogenicity and
Sporulation of Test Organism.
The rice blast pathogen was isolated
and the pathogenicity tested. The patho-
gen (Pyricularia oryzae Cav.) was grown
and maintained in medium containing
rice straw extract and biotin and incu-
bated at constant temperature (280C)
with continuous light.

Laboratory Experiments.

Bioassays were conducted in the labo-
ratory to test the tenacity and residual
effects of different protectant and sys-
temic fungicides with or without ad-
juvants. Fungicides such as Benlate,
Dithane M-45, and Dithane Z-78 (Zineb)
at the rate of 1.5 lbs per hectare per 400
liters and adjuvants Tween 60, Deep
Penetrant and Triton Act M at the rate
of 3 pints per 400 liters were used in
tnese experiments.

Preparation of spore suspension. -
Spore suspension was prepared not more
than 30 minutes before plating at a
concentration of 40,000-50,000 spores
per ml by scraping the surface of 2-
week old cultures using a sterile rubber
policeman.

Seeding and inoculation of the plate.
Two ml of the adjusted spore suspen-
sion was aseptically pipetted to each of
the sterile plates. About 10 ml of the
melted PDA medium which has been
cooled but not solidified was poured
into each plate. The plates were careful-
ly rotated to distribute the agar and
spores evenly, then set aside to congeal.
Preparation of test plants and spraying
of test fungicides. IR-8, a relatively
susceptible variety to rice blast was used
in this experiment. One month after


planting, both leaf surfaces were sprayed
to the point of run-off with fungicide-
adjuvant treatments using Hudson Elec-
tric sprayer.

Preparation of leaf discs. Leaves
from each treatment were detached 24
and 48 hours after spraying. Detached
leaves were divided into 2 groups: one
group was unwashed and the other was
washed in running water for 1 minute to
simulate the effect of weathering in the
different treatments. Washed leaves were
dried, cut into discs using a cork borer
and aseptically placed on seeded agar
using a sterile force with the upper
epidermis facing down. Unwashed leaves
were also cut and placed on the seeded
agar following the same procedure
for washed leaves but without the simu-
lated weathering effects.

Experimental design and gathering of
data. The experiments were conducted
in a randomized complete block de-
sign with 3 replicates and each replicate
with 4 leaf discs. Each treatment was
evaluated after four days incubation by
measuring the diameter of the inhibition
zones and compared with the control.

Nursery and Greenhouse Experiments.

To evaluate the effect of additives
nursery and greenhouse experiments were
conducted using Benlate and Dithane
M45 as fungicides with adjuvants at
the same rates used in the bioassays.
IR-8 was planted in plots with 7
rows, 1 m in length. Each row was
10 cm apart and each plot 1 m apart.
Two border rows were planted around
the plot to serve as guard rows and to
provide the proper environment for
disease development. Approximately 5 g
of seeds were planted in each row.
Ammonium sulfate at the rate of 80
kg N per hectare was applied at planting


Jan. & June, 1975








Philippine Phytopathology


time. Ten days after sowing, the plots
were sprayed with fungicide-adjuvant
treatments using Hudson Electric sprayer.
Inoculation was done late in the after-
noon on the day of spraying, by spread-
ing chopped infected leaves in between
rows one and two, six and seven.

Evaluation of effectiveness of the
different treatments was done two
weeks after inoculation by taking at
random 20 plants from the center
rows of the plots. Average lesion counts
were taken and compared with the
control.

Parallel experiments in pots were also
done in the greenhouse. Each treatment
was accomodated in 12-inch pot and 3
pots were maintained to represent 3
replicates. About 5 g of IR-8 seeds were
pre-germinated and sown in each pot.
Ammonium sulfate at the rate of 1
teeaspoon/pot was also applied at time
of seeding. The same rates of fungicide-
adjuvant treatments as in the nursery
experiment were used.

Diseased leaves were chopped and
spread in each pot to serve as inoculumn
sources on the day of spraying. Evalua-
tion of data was the same as in the
nursery experiment.

Farmer's Field Experiments.
A rice field infected with foliar blast
at Lumban, Laguna was used for the field
trials. The different treatments were
fitted in a randomized complete block
lay-out. First spraying of the different
treatments was done at maximum tiller-
ing stage. The second and third sprayings
were at weekly intervals thereafter.
The rates of application in the field
trial experiments were the same as those
in the nursery, greenhouse and bioassay
experiments.


Sampling was done on each treatment
during harvesting. Five hills were ran-
domly taken from the three rows of each
treatment.

RESULTS

Laboratory tests. Table 1 shows the
data On the inhibition zones ofP. oryzae
from washed and unwashed leaf discs of
rice sprayed 24 hours earlier with fungi-
cide and adjuvant combinations. Fungi-
cide and adjuvant combinations differed
significantly in their inhibition of P.
oryzae.

All treatment combinations were sig-
nificantly different from the control
whether the leaf discs were washed or
unwashed before bioassay was done.
Among the fungicides tested, Benlate
gave significantly larger inhibition zones
compared with Dithane M45 and Di-
thane Z-78. There were no significant
differences observed between Dithane M-
45 and Dithane Z-78.

Bioassay of unwashed leaf discs
generally revealed effectiveness of the
different fungicides due to addition of
various adjuvants. Tween 60 when added
to Benlate exerted larger inhibition zones
compared with Benlate alone. However,
the slight increase in the effectiveness
was not statistically significant. Deep
penetrant and Triton Act M did not
increase the efficacy of Benlate. Bio-
assays indicated that the different ad-
juvants did not significantly increase
the effectiveness of Dithane M45. Tween
60 and Deep Penetrant increased the
effectiveness of Dithane Z-78 but Triton
Act M did not. The increases in the
effectiveness however were not statis-
tically significant compared to Dithane
Z-78 alone.
To determine, the relative tenacity and
penetration or absorption of fungicides


Vol. 11








Fungicide Adjuvants for Rice Blast


Table 1. Inhibition zones (cm) of Pyricularia oryzae 24 hours after spraying chemicals.

Without With
Treatment Washing Treatment Washing

Benlate + Tween 60 2.921 2 Benlate + Tween 60 2.42
Benlate alone 2.48 Benlate + DP 2.30
Benlate + DP 2.38 Benlate 1.68
Benlate + Triton Act M 2.34 Benlate + Triton Act M 1.53
Dithane M-45 1.66 Dithane M-45 + DP 1.21
Dithane A-78 "Tween 60" 1.55 Dithane M-45 + Triton Act M 1.20
Dithane M-45 + Triton Act M 1.54 Dithane M-45 1.12
Dithane M-45 + DP 1.54 Dithane M-45 + Tween 60 1.03
Dithane Z-78 + DP 1.30 Dithane Z-78 + Tween 60 0.75
Dithane Z-78 1.21 Dithane Z-78 + Triton Act M 0.74
Dithane Z-78 + Triton Act M 1.13 Dithane Z-78 + DP 0.63
Dithane M-45 + Tween 60 1.05 Dithane Z-78 0.55
Control 0.00 Control 0.00
Each figure is an average of 3 replicates with 4 leaf discs/replicate.
2Duncan's multiple range test for comparisons among all parts of means;
Means underscored by the same line comprise a group that are not significantly
different at 5% level.


as induced by the adjuvants, leaf discs
were washed to simulate weathering.
Data indicated that Tween 60 and Deep
Penetrant increased the tenacity and
absorptivity of Benlate but Triton Act
M had no effect. All the adjuvants
seemingly increased the effectiveness of
Dithane Z-78. Although there were rela-
tive increases in the absorptivity and tena-
city of the different fungicides as induced
by the adjuvants, the increases were not
statistically significant compared to the
fungicide treatments without adjuvants.

Bioassay results utilizing leaf discs
obtained from rice plants 48 hours after
spraying with fungicide and adjuvant
combinations, are summarized in Table
2.

Results significantly varied among fu-
ficide-adjuvant combinations. All the
treatments increased the inhibition zones
against P. oryzae compared to the con-
trol, whether the leaf discs underwent


a simulated weathering or not. Tween
60 did not significantly increase the
effectiveness of Benlate while Deep
Penetrant and Triton Act M failed to
impart effectiveness to Benlate 48 hours
after spraying. Similarly, all the adju-
vaiis did not significantly increase tlc
effectiveness of D)itlane M45 and
)itliane Z-78 with or without simulated
weathering.


(;reenlhouse and nursery experiments
Results of the greenhouse and nursery
experiments are shown in Table 3. The
data indicate that all the treatments with
Benlate and Dithane M45 with or with-
out adjuvants, significantly reduced the
lesion counts of P. oryzae under green-
house condition compared with the
control. Triton Act M when added to
Benlate and Dithane M45 increased
the effectiveness of these fungicides
against rice blast. Deep Penetrant and
Tween 60 also increased the effectiveness


Jan. & June, 1975









Philippine Phytopathology


Table 2. Inhibition zones (cm) of Pyricularia oryzae 48 hours after spraying chemicals.



Without With
Treatment Washing Treatment Washing

Benlate + Tween 60 2.261 2 Benlate + Tween 60 2.01
Benlate 2.02 Benlate 1.96
Benlate + DP 1.88 Benlate + DP 1.68
Benlate + Triton Act M 1.87 Benlate + Triton Act M 1.64
Dithane M-45 + DP 1.34 Dithane Z-78 + Tween 60 1.26
Dithane M-45 1.32 Dithane M-45 1.24
Dithane Z-78 1.26 Dithane M-45 + DP 1.08
Dithane Z + DP 1.21 Dithane M-45 + Triton Act M 1.07
Dithane Z-78 + Tween 60 1.12 Dithane Z-78 + Triton Act M 1.06
Dithane M-45 + Tween 60 1.11 Dithane Z-78 1.00
Dithane M-45 + Triton Act M 1.08 Dithane Z-78 + DP 0.86
Dithane Z-78 + Triton Act M 0.97 Dithane M-45 + Tween 60 0.81
Control 0.00 Control 0.00

SAverage of 3 replicates with 4-leaf discs/relicate
2Duncan's multiple range test for comparisons among all parts of means;
Means underscored by the same line comprise a group that are not significantly
different at 5% level.

Table 3. Average counts of Pyricularia oryzae-induced lesions under greenhouse and
nursery conditions.


Greenhouse
Experiment


Treatment


Field
Experiment


Treatment


Benlate + Triton Act M
Benlate
Dithane M-45 + Triton Act M
Benlate + Tween 60
Dithane M-45 + Tween 60
Benlate + DP
Dithane M-45 + DP
Triton Act M
Dithane M-45
Tween 60
DP
Control


221 2
26
27
29
29
31
31
32
35
36
43
49


Dithane M-45 + Tween 60
Benlate + Triton Act M
Benlate + DP
Benlate + Tween 60
Dithane M-45 + Triton Act M
Benlate
Dithane M-45
Dithane M-45 + DP
Tween 60
DP
Triton Act M
Control


5.83
6.66
7.23
8.30
8.94
10.19
11.16
14.26
29.13
29.22
29.55
45.01


SAverage of 3 trials, each trial was replicated 3 times with 20 plants per treatment.
2Duncan's multiple range test for comparisons among all pairs of means;
Means underscored by the same line comprise a group that are not significantly
different at 5% level.


Vol. 11









Fungicide Adjuvants for Rice Blast


of Dithane M45 but not that of Benlate.
The increases however, were not statis-
tically significant.
Similar results have been observed
in the field experiments. All the fungi-
cides with or without adjuvants reduced
the incidence of rice blast compared
with the control. Adjuvants applied alone
did not differ from the control. Tween 60
and Triton Act M increased the effective-
ness of both fungicides. Deep Penetrant
also increased the effectiveness of Benlate
but not that of Dithane M45.

Farmer's field experiments. The
results of experiments in the farmer's
field are presented in Table 4. Analysis
showed that Benlate and Dithane M45
with or without adjuvants reduced the
neck rot incidence and percent empty
grains. However, the reductions were


not statistically significant. No signifi-
cant increase in the yield was noted.
Likewise, the different adjuvants did
not significantly increase the effective-
ness of the fungicides at actual field con-
dition.

DISCUSSION

Results of our laboratory bioassay
confirmed that the "DP" did not signifi-
cantly increase the efficacy of the fungi-
cides used with liquid fertilizers and other
fungicide solutions (San Diego 1970;
Donato, et. al. 1970; Canoy, 1969 and
Schultz and Dalmacio, 1970). The data
support the contention of Canoy, (1970)
and Alimagno and Ramos (1970) that DP
slightly enhanced the absorption of
chemical substances. Its efficiency is
comparable to other adjuvants used in


Table 4. Percent neck rot, emptygrains and threshing percentage.'


Percent Percent
Threshing3 Empty Actual
Treatment Neck Rot2 (%) Grains 4 Yield'

Benlate 4.66 91.96 15.06 165.16
Benlate + DP 3.33 56.17 18.00 160.66
Benlate + Tween 60 9.33 50.58 19.53 147:16
Benlate + Triton Act M 4.00 90.11 19.46 142.83
Dithane M-45 8.00 91.38 12.66 148.30
Dithane M-45 + DP 11.33 85.12 22.00 145.66
Dithane M-45 + Tween 60 6.66 89.24 21.73 142.50
Dithane M-45 + Triton Act M 8.66 90.21 19.73 152.16
Control 10.00 86.58 25.46 148.33

'Average of one trial with 3 replicates/treatment
2 percent Neck Rots = No. of neck rots counted X 100
50
3 Percent Empty grains No. of empty grains counted X 100
Percent Empty grains = -*-----_-___ ----- X 100
500
4 Threshing percent = 100 loss of weight
Percent loss of weight uncleaned grain cleaned grain 100
Percent loss of weight = X 100
uncleaned grain
5 Actual yield = actual weight of the uncleaned grain after threshing


Jan. & June. 1975








Philippine Phytopathology


these experiments under laboratory con-
dition against P. oryzae.

In greenhouse and field experiments,
the data also indicated that DP and other
adjuvants did not significantly increase
the efficacy of Benlate and Dithane M-45
in the control of foliar blast compared
to those treated with fungicides alone.
These results parallel with the results
reported by Quebral, Quisumbing and
Rasco (1970) where DP did not enhance
the efficacy of Benlate, Dithane M45
and Captan in the control of Leveillula
taurica (Lev.) Arn. and did not corro-
borate the findings of Schultz and Dal-
macio (1970) that DP greatly enhanced
the effectivity of chloroneb as foliar
spray against downy mildew of corn.
These results indicate that DP may be
selective in enhan ing toxicity of fungi-
cides.

The non-efficacy of adjuvants as
additives to fungicides against rice blast
under greenhouse and field conditions
may be due to the frequent rains that
occurred during the experiments.

Under grecenhouse and field con-
.ditions, it was difficult to control the
effects of the environment, since the ex-
periments were conducted monthly from
March-December, 1974. Environmental
conditions monitored showed that ave-
rage temperature ranged from 78-


830F while the percent RH and amount
of rainfall ranged from 74-90% and 0.11-
19.72 in respectively. This may be one
of the reasons why the results of the
experiments in the laboratory, green-
house and nursery, and in the farmer's
field did not corroborate with one
another.
Experiments in the farmer's field
revealed that the fungicides failed to
control rice blast. The adjuvants also
failed to significantly increase the effi-
cacy of the fungicides as in the laboratory
and greenhouse experiments. The expe-
riments in the farmer's field had limita-
tions. There are many interactions of
factors which cannot be controlled. The
Lumban's weather condition was very
favorable for the development of the
pathogen such that at high inoculum
densities effectiveness of fungicides and/
or fungicides-adjuvants can not be as-
certained.
The over all results of the experiments
indicated that with the fungicides,
methodology and rates of DP appli-
cations used, DP is another chemical
which could serve as an adjuvant. It
is comparable to other commercial adju-
vants as wetting agent and or spreader-
sticker for fungicides. It is suggested
that further verification of the efficacy
and selectivity of DP be conducted using
different approaches and methodologies.


LITERATURE CITED

ALIMAGNO, B.V. and B. B. RAMOS. 1971. Preliminary study: The effect of "Deep
Penetrant" on the' rate of uptake of phosphorous in plants using P32 as tracer. I.
Hoagland solution as medium. Report released to Agricultural Research Center,
NIST by the Philippine Atomic Research Center. 1971.
BUNOAN, J.C., T. EUGENIO, L.M. VILLEGAS, R. FEUER, and S. K. DE DATTA. The
Philippines Recommends for Rice 1970. pp. 18-19. NFAC.
BUNOAN, J.C. Jr. 1971. Field demonstrations on the use of DP (Deep Penetrant) as an
additive for applied fertilizer in increasing yield of lowland rice in Pangasinan Pro-
vince. In A Solution to Our Shortage. P.O. Box 2627, Manila.
CANOY, C.S. 1969. Results of test conducted with "Deep Penetrant." NIST Report.
1970.


Vol. 11








Jan. & June, 1975 Fungicide Adjuvants for Rice Blast 79

DONATO, R.T., J.M. MANGLICMUT and A. GABUCAN. 1970. A preliminary report on
the effect of "Deep Penetrant" on the yield of BPI-48. Report of Bureau of Plant
Industry released to NFAC, DANR.
NATIONAL FOOD AND AGRICULTURAL COUNCIL, DANR. 1972. Rice production
program for fiscal year, 1971-1972. Pamphlet 1-44.
QUEBRAL, F.C., E.C. QUISUMBING and E.T. RASCO. 1972. The effect of fungicides
sprayed alone and in combination with "Deep Penetrant" on VC II I tomatoes.
Proceedings of the Annual SAVI meeting. CLSU, April, 1972.
SAN DIEGO, G. 1970. Preliminary study on the feasibility of "Deep Penetrant" as addi-
tive to fertilizer on its efficacy and response to lowland rice grown under Bigan
Clay Loam. In a Solution to Our Shortage. Prepared by Johny Lilio: Chandler"
Enterprise P.O. Box 2627 Manila.
SCHULTZ, O.E. and S.C. DALMACIO. 1970. Investigation on chemical control of Philip-
pine downy mildew of corn. Paper presented at the Intensified Corn Program Semi-
nar, held at the UPCA, College, Laguna, March 22-27, 1971.







Phil. Phytopathol. 11:80-90
Jan. & June, 1975

ASSESSMENT OF RICE YIELD LOSS DUE TO
BACTERIAL LEAF BLIGHT

DELFIN B. LAPIS and SAWATDEE LIANSUTHISAKON

Assistant Professor and Graduate Student, respectively, Department of Plant Patho-
logy, College of Agriculture, UP at Los Bafios. College, Laguna, Philippines.
This study was supported in part by Rice Project 3.3 and SEARCA.

ABSTRACT

Yield losses in IR-8, C4-63(G) and IR-20 due to bacterial leaf blight were deter-
mined at 60, 75 and 90 days after sowing and at 3 disease intensity levels of 25, 50 and
75 per cent. The average yield losses in IR-8 were 10.77 and 25.87 per cent, in C4-63(G),
8.01 and 18.76 per cent and in IR-20, 3.31 and 13.82 per cent for the dry and wet sea-
sons, respectively. The linear regression of yield on disease intensity in the 3 varieties
were statistically significant at both seasons of planting. The average decrease in yield of
IR-8 were 11.68 and 8.76 g, in C4-63(G), 7.67 and 6.16 g and in IR-20, 3.81 and 4.34 g
per 0.75 m2 for each unit increase in disease intensity for the dry and wet seasons, res-
pectively.


One of the most serious diseases which
cause substantial losses in rice yield is
the bacterial leaf blight, incited by
Xanthomonas oryzae (Uyeda et Ishiya-
ma) Dowson. This disease occurs in
most of the rice growing areas of the
Philippines and in most of the Southeast
Asian Countries. The disease causes
decrease in yield of rice even on the
resistant varieties. A virulent type of
blight organism caused considerable yield
losses varying from slight to very serious;
its most destructive form, "Kreseck",
kills young plants completely.
The high yielding varieties, IR-8
and Taichung Native I, which were
recommended for the expanded rice
production program in many countries,
are susceptible to this disease. The di-
sease has been recognized as one of the
most important rice diseases in Japan
since 1884. It was reported to occur in
50,000 to 60,000 hectares of rice fields
in 1940, increasing to about 300,000
to 400,000 hectares 20 years later. Yield
losses in infested field ranged from 20
to 30 per cent depending on the season,


variety, type and time of infection.
It also caused poor development and low-
quality grains, decrease in soluble nitro-
genous substances, and increase in crude
protein because the grain does not mature
completely (Tagami and Mizugami,
1962).
Bacterial leaf blight is a serious foliage
disease of both upland, rainfed and low-
land rice in the Philippines, usually more
severe in upland varieties. Silva (1968)1
reported that the average rice yield loss
due to this disease from 1000 grain
weights was 33.1%.
In India, bacterial leaf blight of rice
is also recognized as one of the major
diseases of rice plant. Losses in yield
vary from 6 to 60% depending upon the
severity and stage of infection (Srivas-
tava et al. 1966). Ray and Scngupta
(1970) reported that it has been a serious


'Silva, J. P., Some studies on bacterial
leaf blight disease of rice in the Philip-
pines. Paper presented at the IRRI
seminar, June, 1968.







Yield Loss Due to Blight


disease on some of the high yielding
exotic varieties particularly Taichung (N)
1. The affected plants showed a high
proportion of chaffy grains. The total
number of glumes was 33.2% higher
in affected tillers than in healthy tillers.
Similarly the weight of 1000 grains was
4.8 grams less in affected tillers.

Jolavicharana (1958) and Ou (1963)
reported that bacterial leaf blight
commonly occur in the rice field in Thai-
land. In Sri Lanka, there was a severe
outbreak of "Kreseck" phase of bacte-
rial leaf blight of rice. A large number
of plants were found dying in about 2
weeks after transplanting, the incidence
or percentage often exceeding 50% and
sometimes attaining 90% (Shigemura
and Tabei, 1969).

Yield losses of rice caused by bacterial
leaf blight has become a serious problem
in most rice growing countries in recent
years but the actual yield losses have
not been determined critically. Hence,
this study was conducted in lowland rice
field of the Central Experiment Station,
University of the Philippines at Los
Bafos, College, Laguna from July, 1972
to April, 1974 in order to determine:
1) the rice yield losses caused by X.
oryzae on susceptible, intermediate and
resistant varieties; and 2) the regression
in different disease intensities at the
various stages of growth of the rice plants
for dry and wet seasons.

MATERIALS AND METHODS

Planting and care of plants. Three
non-seasonal rice varieties IR-8, C4-
63(G) and IR-20, susceptible, inter-
mediate and resistant varieties respec-
tively, were raised by wet bed method
and transplanted into the lowland rice
field at the rate of one seedling per hill


and a distance of 20 x 25 centimeters
during the 1972 dry and 1973 wet
seasons. Experimental plants were given
the recommended lowland operation such
as weeding, pest control, and other cul-
tural practices.

Fertilization. The rate of 120 kg
nitrogen per hectare as ammonium sul-
fate was used. Split-application was em-
ployed; 1/2 of the total amount was ap-
plied at planting and the other half 35-40
days after.

Preparation of inoculum. A virulent
isolate of X. oryzae (PXO 25) was isola-
ted. Its pathogenicity was tested and cul-
tures were maintained in Wakinoto's
medium. Preparation of inoculum was
patterned after Dalmacio and Exconde
(1967) in their study of the host range of
X oryzae in the Philippines. and the
method of inoculation was that of Muko
and Yoshida (1951) with slight modifi-
cation of the pricking meedles (Fig. 1.).

The three uppermost leaves of the ex-
perimental plants in each treatment,
except the control, were inoculated at
60, 75 and 90 days after sowing (DAS)
with the desired disease intensities of
25, 50 and 75 per cent respectively
(Fig. 2). To obtain the desire disease
intensities, inoculum concentrations were
varied at the rate of 107, 10" and 109
cells per ml, respectively.

Disease-free check (O intensity) was
maintained by regularly spraying anti-
biotics and fungicides to ward off bacte-
rial and/or fungal diseases.

Experimental design. The experiment
was carried out in a split-split plot ex-
periment in a randomized complete
block design (RCBD) with four replicates.
Variety was the main plot, stage of ino-
culation were the sub-plots.


Jan. & June, 1975









Philippine Phytopathology


Fig. 1. Multiple needles used for inoculation of X. oryzae on the leaves.
A modification of Muko and Yoshida's pricking needles.


Fig. 2. Disease Intensity Scales, 0 = cheek,
1 = 25 percent, 2 = 50 percent and
3 = 75 percent leaf area infested by
the disease.


Vol. II








Yield Loss Due To Blight


Gathering of data. Harvesting was
done when 80-90 per cent of the grains
were ripe. Data on grain yield was col-
lected from all the plants within each
treatment. After harvesting, the grains
were dried under the sun until the
moisture content was about 13 per cent
and then their respective weights were
taken. The moisture content was deter-
mined by a Steinlite Moisture tester.

RESULTS

The disease caused discernible losses
in grain yield of the 3 rice varieties,
There were appreciable losses in yield
due to the time of inoculation at 3
stages of plant growth in each variety.

The grain yield losses on the 3 rice
varieties at the different inoculation
times were: on IR-8, 11.99, 11.31 and
9.23; and 26.33, 23.43 and 26.19%
(Table 1): on C4-G3 (G), 10.08, 8.81 and
5.22; 13.63, 19.60 and 21.33% (Table 2);
and on IR-20 they were 4.29, 3.37 and
2.37; 10.49, 18.32 and 15.96% (Table 3)
for the dry and wet seasons, respectively.



Table 1. Yield losses due to X. oryzae on
season of planting.


The grain yield losses on inoculation
at 60 days after sowing were 7.11, 8.92
and 10.43% at disease intensities 1, 2
and 3 in the dry season, respectively.
During the wet season, computed yield
losses at each disease intensities were
15.44, 18.27 and 19.31% respectively.
The average losses were 8.82 and 17.67%
for the dry and wet seasons, respectively.
Significant reduction in yield was noted
between disease intensities 1 and 2 or 1
and 3 in the dry season. The yield losses
were statistically significant in the dry
season and insignificant in the wet
season (Table 4).

The grain yield losses on inoculation
at 75 days after sowing were 5.48, 10.69
and 6.87% at disease intensities 1, 2
and 3 respectively, during the dry season.
During the wet season, computed yield
losses at each disease intensities were
18.31, 19.88 and 19.47% for the dry
and wet seasons, respectively. The average
losses were 7.68 and 19.22% for the dry
and wet seasons, respectively (Table 5).
Losses in yield on inoculation at 90
days after sowing were 5.02, 6.01 and



IR-8 at indicated stages of inoculation and


Dry Season 1972 Wet Season 1973
Stage of
inoculation Grain yield Loss % Grain yield Loss %
(g/0.75 m2) (g/0.75 m2)

02 303.63 104.70
60 DAS 267.21 11.99 77.11 26.33
0 310.25 100.88 -
75 DAS 275.17 11.31 77.24 23.43
0 309.25 106.68
90 DAS 281.34 9.23 78.73 26.19

L Each figure represents an average of 3 disease intensity levels.
2 Disease-free check (control)


Jan. & June, 1975








Philippine Phytopathology


Table 2. Yield losses due to X. oryzae on C4-63(G) at indicated stages of inoculation and
season of planting.


Dry Season 1972 Wet Season 1973

Stage of Grain yield1 Loss % Grain yield Loss %
inoculation (g/0.75 m2) (g/0.75 m2)

02 276.00 93.80 -
60 DAS 248.17 10.08 79.13 13.63
0 281.17 101.55 -
75 DAS 256.92 8.81 81.65 19.60
0 277.15 100.48 -
90 DAS 262.67 5.22 79.15 21.23

SEach figure represents an average of 3 disease intensity levels.
2 Disease-free check (control)

Table 3. Yield losses due to X. oryzae on IR-20 at indicated stages of inoculation and
season of planting.


Dry Season 1972 Wet Season 1973
Stage of Grain yield' Loss % Grain yield Loss %
inoculation (g/0.75 m2) (g/0.75 m2
02 287.75 99.80 -
60 DAS 275.38 4.29 89.33 10.49
0 291.50 -100.03
75 DAS 281.67 3.37 81.71 18.32
0 286.00 98.98 -
90 DAS 279.21 2.37 83.10 15.96

1Each figure represents an average of 3 disease intensity levels.
2 Disease-free check (control)


7.46% at the disease intensities 1, 2 and
3, respectively in the dry season. In the
wet season, computed yield losses at 3
disease intensities were 20.49, 21.49
and 23.69% respectively. The average
losses in yield were 6.16 and 21.89%
for the dry season and wet seasons,
respectively (Table 6).

Grain yield losses of IR-8. The
effect of X. oryzae on grain yield was


computed in terms of grain weight.
Losses in yield were 8.83, 11.41 and
12.06 per cent at disease intensities,
1, 2 and 3, respectively during the dry
season. In the wet season, calculated
yield losses at 3 disease intensities were
22.82, 27.83 and 26.95% respectively.
The average losses were 10.77 and
25.87% during the dry and wet seasons,
respectively. The yield losses were highly
significant in the dry season but insigni-


Vol. 11








Yield Loss Due To Blight


Table 4. Yield losses due to X. oryzae inoculated 60 days after sowing at indicated
disease intensities and seasons of cropping.

Dry Season 1972 Wet Season 1973
Disease Grain yield' Loss % Grain yield Loss %
intensity (g/0.75 m2 ) (g/0.75 m2)

0 289.13a -99.43a -
25 268.46**b 7.11 84.08**b 15.44
50 263.33**c 8.92 81.28**b 18.27
75 258.96**cd 10.43 80.23**bd 19.31
Average 269.97 8.82 86.26 17.67

1Each figure represents an average of 3 varieties with 4 replicates each. Within vertical
column, figures followed by different letters are significantly different (1%) by Dun-
can's Multiple Range Test.
** Significant at 1% level of probability.


Table 5. Yield losses due to X. oryzae inoculated
sease intensity and season of cropping.


75 days after sowing at indicated di-


Dry Season 1972 Wet Season 1973
Disease Grain yield1 Loss % Grain yield Loss %
intensity (g/0.75 m2) (g/0.75 m2)

0 294.50a 100.82a -
25 274.42**b 5.48 82.36**b 18.31
50 269.29**bc 10.69 80.78**bc 19.88
75 270.38**bd 6.87 81.19**bd 19.47
Average 277.15 7.68 86.29 19.22

1 Each figure represents an average of 3 varieties with 4 replicates. Within vertical co-
lumn, figures followed by different letters are significantly different (1%) by Duncan's
Multiple Range Test.
** Significant at 1% level of probability.


ficant in the wet season. Significant re-
duction in yields were noted between
disease intensities 1 and 2 or 1 and 3
(Table 7).

To find the relationship between
disease intensity and yield loss, a regres-
sion line was fitted on the data, con-
sidering yield loss (Y), as the dependent
variable, and disease intensity (X) as
the independent variable. This relation-
ship was illustrated in the susceptible


variety IR-8 -grown during the wet and
dry seasons (Fig. 3). The line obtained
is Y = 300.59 11.68 X and Y = 97.45
- 8.76 X for the dry and wet seasons res-
pectively. The regression coefficient was
tested by "T" test and found signifi-
cant at 1% level of probability, thus
showing that the increase of yield losses
during both seasons was due to-the
increased intensities of the disease and
not due to the times of inoculation.
Grain yield losses of C4-63 (G). -


Jan. & June, 1975








Philippine Phytopathology


Table 6. Yield losses due to X. oryzae inoculated 90 days after sowing at indicated di-
sease intensities and seasons of cropping.


Dry Season 1972 Wet Season 1973
Disease Grain yield' Loss % Grain yield Loss %
intensity (g/0.75 m2 ) (g/0.75 m2 )

0 291.00a 102.88a -
25 276.13**b 5.02 81.80**b 20.49
50 273.20**bc 6.01 80.67**bc 21.49
75 273.92**bd 7.46 78.50**bd 23.69
Average 278.56 6.16 85.96 21.89

SEach figure represents an average of 3 varieties with 4 replicates. Within vertical co-
lumn, figures followed by different letters are significatnly different (1%) by Duncan's
Multiple Range Test.
** Significant at 1% level of probability.


Table 7. Yield losses due to X. oryzae in IR-8 at indicated disease intensities and seasons
of planting.


Dry Season 1972 Wet Season 1973

Disease Grain yield' Loss % Grain yield Loss %
intensity (g/0.75 m2) (g/0.75 m2)

0 307.70a 104.80a
25 280.54**b 8.83 80.89**b 22.82
50 272.58**c 11.41 75.63**bc 27.83
75 270.58**cd 12.06 76.64**bd 26.95
Average 282.85 10.77 84.49 25.87


Each figure represents an average of 3 stages of inoculation and 4 replicates. Within
vertical column, figures followed by different letters are significantly different (1%)
by Duncan's Multiple Range Test.
** Significant at 1% level of probability.


- The effect of X. oryzae on grain yield
was calculated in terms of grain weight.
Losses in yield were 6.78, 8.55 and 8.69%
at disease intensities 1, 2 and 3 respec-
tively during the 1972 dry season. During
the 1973 wet season calculated yield
losses at 3 disease intensities were 18.64,
15.92 and 21.73% respectively. The


average losses were 8.01 and 18.76%
in the dry and wet seasons, respectively
(Table 8).

The relationship between grain yield
and disease intensity is shown in Fig.
4. The regression line fitted on the data
representing yield losses due to disease


Vol. 11








Yield Loss Due To Blight


y On SeaSON
y, O.9- 1.6BX
-j-


ITz


WET sBe9CN
= S *zx


s=ASe i4nrN-871


Fig. 3. Relationship between grain yield
Y and disease intensity X, on
IR-8, 1972 dry season and 1973
wet season.





intensities gave the regression equation
Y =273.07 7.67 X and Y =93.97
- 6.16 X for the dry and wet seasons
respectively. The "T" test showed the



Table 8. Yield losses due to X. oryzae in
seasons of planting.


regression coetticient was significant at
the 1% level of probability, showing that
increased intensities of the disease during
both seasons significantly increased yield
losses.
Grain yield losses of IR-20. The
effect of X. oryzae on grain yield losses
was calculated in terms of grain weight.
Losses in yield were 3.03, 3.41 and
3.49% at disease intensities 1, 2 and
3, respectively, in the dry season 1972.
In the wet season, 1973, calculated yield
losses at 3 disease intensities were 12.48,
15.48, and 13.51% respectively. The
average losses were 3.31 and 13.32%
in the dry and wet seasons, respectively
.(Table 9).
The relationship between grain yield
and disease intensity is shown in Fig.
5. The regression line fitted on the data
representing yield losses due to disease
intensities gave the regression equation
Y = 287.53 3.18 X and Y =95.78
4.34 X for the dry and wet season,
respectively. The "T" test showed that
the regression coefficient was significant
at 5 and 1% level of probability, for the
level dry and wet seasons, respectively,



C4-63 (G) at indicated disease intensities and


Dry Season 1972 Wet Season 1973

Disease Grain yield Loss Grain yield Loss
intensity (g/0.75 m2) % (g/0.75 m2) %

0 278.50 98.60a
25 259.63**b 6.78 80.22**b 18.64
50 254.67**bc 8.55 82.90**bc 15.92
75 254.30**bd 8.69 77.17**bd 21.73
Average 261.78 8.01 84.73 18.76

SEach figure represents an average of 3 stages of inoculation and 4 replicates. Within
vertical column, figures followed by different letters are significantly different (1%)
by Duncan's Multiple Range Test.
** Significant at 1% level of probability.


Jan. & June, 1975


9









Philippine Phytopathology


DA9 SEASON
Y 7aor rTTx







WET W6ASON
y t.9r 6 i6K


VXAWf INrTEsITr


Fig. 4. Relationship between the grain
yield Y and disease intensity X
on C4-63 (G), 1972 dry season
and 1973 wet season.


Y ; 2T5. 5.81 x




0 2001


WET SEASON
3 0I Y; 9S:7S -4.54)x
S 6-- --




1 2. 3
isuEse INTE5SIT!


Fig. 5. Relationship between the grain
yield, Y and disease intensity X
on IR-20, 1972 dry season and
1973 wet season.


Table 9. Yield losses due to X. oryzae in IR-20 at indicated disease intensities and seasons
of planting.


Dry Season 1972 Wet Season 1973

Disease Grain yield1 Loss Grain yield Loss
intensity (g/0.75 m2) % (g/0.75 m2) %

0 288.42a 99.60a
25 279.67**b 3.03 87.17**b 12.48
50 278.58**bc 3.41 84.18**bc 15.48
75 278.33**bd 3.49 36.14**bd 13.51
Average 281.25 3.31 89.27 13.82

Each figure represents an average of 3 stages of inoculation and 4 replicates. Within
vertical column, figures followed by different letters are significantly different (1%)
by Duncan's Multiple Range Test.
** Significant at 1% level of probability.


showing that increased intensities of the
disease both seasons significantly in-
creased yield losses.


DISCUSSION

The results show that bacterial leaf
blight of rice can decrease the yield
potential of almost all varieties. The


disease was capable of causing high losses
on yield, particularly on the susceptible
IR-8 and intermediate C4-63(G). Losses
in yield of the resistant IR-20 were not as
high as those in the other two varieties.
Such losses further demonstrated that
yield losses caused by this disease depend
not only on the susceptibility of the rice
plants, but also on the stage the plants


.1 Vol. 11


100 I








Yield Loss Due To Blight


got infected, severity of infection and
cropping season.

Although no actual counts were made
on the number of unripe and empty
grains, the reduction in yield of IR-8
was probably due to the increase in the
number of partially filled or empty
grains. Tagami and Mizukami (1962) re-
ported yield losses in severely infected
rice range from 20 to 30% depending
upon the season, variety, type and time
of infection. The reduction in the grain
yield was due to incomplete ripening and
damaged kernels.

Since there was no interaction in yield
between stages of inoculation and rice
varieties, losses in yield depended on the
interaction between the disease inten-
sities. Reduction in yield was increased
when there was an increase in the disease
intensity. Differences in yield losses
were significant in the dry season but
insignificant during the wet season.


Disease intensities greatly influenced
reductions in yield of IR-8 and C4-63(G)
in both seasons. As the intensity in-
creased, the yield losses also increased.
Exconde et al. (1973) reported that
bacterial leaf blight of rice can greatly
affect yield potentials of BPI-76 with
an average yield reduction of 22.5 and
7.2% for the wet and dry seasons,
respectively. The average losses in BPI
76-13 were 9.5 and 1.8% for the wet
and dry seasons, respectively. No signi-
ficant loss in yield was recorded on
C4-63(G) which may be due to the
resistance of this variety to strain B
used in this experiment. In all three


varieties no significant interaction was
obtained between the time of inocula-
tion and disease intensity. Aldrick
(1972), reported that the yield losses
in Australia as measured in 1000 gram-
weights were reduced by almost 75%
in artificially inoculated IR-8 and by
33% in naturally infected Taichung
(N) 1. Ou (1972) reported that yield
losses on artificially inoculated sus-
ceptible IR-8 and moderately resistant
Tainan 8 to be 75 and 45% respectively.
Ou et al. (1972) estimated yield losses
due to bacterial leaf blight of rice in the
form of per cent of total leaf areas in-
fected among the three upper leaves of
each tiller. They reported that when the
leaf areas damaged reach 50 and 100%
of the leaf, losses were about 25 and 50%
respectively.

Varietal resistance to bacterial blight
is rather difficult to ascertain because
the strains differ in their pathogenicity
patterns. Based on the studies of an
international collection of isolates,
Buddenhagen et al. (1969) demonstra-
ted wide differences in virulence, the
isolates from tropical Asia being more
virulent.


Our results showed the yield losses
from infected plants during the dry
season may not be as high as losses in
the wet season; the average losses in
yield of the three varieties were 7.36
and 19.84% in the dry and wet seasons,
respectively. The yield loss of the re-
sistant IR-20 during the wet season
suggests that bacterial leaf blight could
also affect the yield of this variety under
severe disease pressure.


Jan. & June, 1975









Philippine Phytopathology


LITERATURE CITED

ALDRICK, S.J. 1972. Diseases of rice. Australian Rice Res. Conf. February 4, 1972
12-20.
BUDDENHAGEN, I.W., J. SILVA and S.H. OU. 1969. First year report of the coopera-
tive project on comparison of the virulence of X. oryzae strains from different
Asian countries. Univ. of Hawaii, Honolulu.
DALMACIO, S.C. and O.R. EXCONDE. 1967. Host range of Xanthomonas oryzae in
the Philippines. Phil. Agric. 51: 283-289.
EXCONDE, O.R., O.S. OPINA and K. PHANOMSAWARN. 1973. Yield losses due to
bacterial leaf blight of rice. Phil. Agric. 57: 128-140.
JOLAVICHARANA, K. 1958. Outbreak and new records, Rev. Appl. Mycol.
MUKO, H. and K. YOSHIDA. 1951. A needle inoculation method for bacterial leaf
blight disease of rice. Ann. Phytopathol. Soc. Japan 15:179.
OU, S.H. 1963. Rice blast disease and breeding for its resistance in Thailand with notes
on other rice diseases. FAO Report, 43 pp.
OU, S.H. 1972. Rice diseases. Commonwealth Mycological Inst. Kew Surrey, England.
368 p.
RAY, P.R. and T. K. SENGUPTA. 1970. A study on the extent of loss in yield of rice
due to bacterial blight. Indian Phytopathol. 23:713-714.
SHIGEMURA, G. and H. TABEI. 1969. Measurement to control kreseck disease of rice
in Ceylon. International Rice Commission, Newsletter 18: 12-23.
SHRIVASTAVA, D.N., Y.P. RAO, and J.C. DURGAPAL. 1966. Can Taichung native I
stand up to bacterial leaf blight? Indian Phytopathol. 16:75.


Vol. 11






Phil. Phytopathol. 11:91-92
Jan. & June, 1975

SAMPAGUITA YELLOW RINGSPOT MOSAIC




D.A. BENIGNO, M.A. FAVALI-
HEDAYAT and M.L. RETUERMA1


Sampaguita (Jasminum sambac (L.)
Ait. the national flower of the Phil-
ippines has, until recently, been ob-
served to be infected by a virus.
Electron microscopic investigation of
crude extracts, obtained from diseased
plants, revealed the presence of slightly
flexible filamentous rod-like particles
measuring from 700-750 nm (Plate IA).
Ultrathin sections of diseased leaves also
revealed similar rod-like particles in the
cytoplasm (CY) of most mesophyll cells
in aggregates of about 20-30 particles
arranged parallel to each other (Vi).
Pin-wheel-like inclusion (PW), a diag-
nostic feature for viruses of Brandes'
potato virus Y group, was, likewise, ob-


served appearing consistently along the
cell wall (Plate IB).
Attemtps to transmit the virus me-
chanically by sap and by aphids failed.
However, it was transmitted by grafting
and by the whitefly, Bemisia tabaci Genn.
The first discernible symptom observed
is usually the appearance of bright yellow
irreg. early circular spots on young leaves
measuring about a millimeter or more
in diameter (Plate IIA). Later, the ring-
spot enlarge assuming irregular line pat-
tern symptoms making the whole leaf
appear more yellow than green (Plate
IIB). It is often observed also that in-
fected plants has a tendency to recover.


Plate I. Slightly flexible long rod-like particles of sampaguita yellow ringspot
mosaic virus obtained from crude extracts (A) and from ultra-thin
sections (B).

'Assistant Professor, Visiting Scientist and Graduate Student respectively, Depart-
ment of Plant Pathology, College of Agriculture, University of the Philippines at
Los Bafios.










92 ----PHiippinbD P42iytopathology Vol. 11
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