• TABLE OF CONTENTS
HIDE
 Introduction
 Transmission
 Symptom expression
 Papaya ringspot virus type W
 Watermelon mosaic virus 2
 Cucumber mosaic virus
 Figures






Title: Aphid-transmitted viruses of curcubits in Florida
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Full Citation
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Permanent Link: http://ufdc.ufl.edu/UF00066842/00001
 Material Information
Title: Aphid-transmitted viruses of curcubits in Florida
Series Title: Florida Cooperative Extension Service Circular 1184
Physical Description: Book
Language: English
Creator: Kucharek, Tom
Purcifull, Dan
Affiliation: University of Florida -- Florida Cooperative Extension Service -- Institute of Food and Agricultural Sciences
Publisher: Florida Cooperative Extension Serive, Institute of Food and Agricultural Sciences, University of Florida
Publication Date: 1997
 Subjects
Subject: Curcurbitaceae   ( lcsh )
Aphididae   ( lcsh )
Plant diseases   ( lcsh )
Spatial Coverage: North America -- United States of America -- Florida
 Notes
Funding: Florida Historical Agriculture and Rural Life
 Record Information
Bibliographic ID: UF00066842
Volume ID: VID00001
Source Institution: Marston Science Library, George A. Smathers Libraries, University of Florida
Holding Location: Florida Agricultural Experiment Station, Florida Cooperative Extension Service, Florida Department of Agriculture and Consumer Services, and the Engineering and Industrial Experiment Station; Institute for Food and Agricultural Services (IFAS), University of Florida
Rights Management: All rights reserved by the source institution and holding location.

Table of Contents
    Introduction
        Page 1
    Transmission
        Page 2
        Page 3
    Symptom expression
        Page 4
    Papaya ringspot virus type W
        Page 5
    Watermelon mosaic virus 2
        Page 6
    Cucumber mosaic virus
        Page 7
        Page 8
        Page 9
    Figures
        Page 10
        Page 11
        Page 12
        Page 13
        Page 14
Full Text



Circ-1184


Aphid-Transmitted Viruses of Cucurbits in

Florida
Tom Kucharek and Dan Purcifull, Respectively Professor and, Professor, Plant
Pathology Department, University of Florida, Gainesville, 32611. April 1997
(Revised 2001)
Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Christine Waddill, Dean


Cucurbit crops (Cucurbitaceae, cucumber &
gourd family) are of major importance to the
state of Florida and its agricultural communi-
ties. For the 1994-1995 marketing year, water-
melons, squash, and cucumbers were valued
at $146,079,000 in Florida. This was 9.9 percent
of the total value of vegetables at the farm-gate
and does not include other cucurbits such as
cantaloupes and pumpkins. Florida produces
more watermelons than any other state. In ad-
dition to several fungal diseases, viral diseases
have strongly interfered with the establishment
of a pumpkin industry in Florida.

For at least the past three decades, the most
important impediment to consistent production
of squash, watermelons, and some other cucur-
bits in Florida, has been the annual occurrence
of aphid-transmitted viruses. Both yield and
fruit quality have been reduced significantly by
these viruses. Growers constantly ask about the
availability of useable control measures. Inter-
estingly, cucumber has not incurred many vi-
ral problems for the past three decades or so,
except where some exotic varieties were used
in the greenhouse or in the field.

On a world-wide basis, more than 50 viruses
and four viroids have been reported to infect at
least one member of the cucurbit family. At least
half of the viruses are economically important
pathogens of commercial cucurbits. Currently,
five aphid-transmitted viruses of cucurbits have


been found in Florida. These viruses are com-
posed of a ribonucleic acid core encapsulated
by a protein "coat". Table 1 summarizes the
current nomenclature of these viruses.

Table 1. Aphid-transmitted viruses found
naturally in Florida-produced cucurbits.
Virus Acronyms
Paypaya ringspot virus PRSV-W(WMV 1)
type W
Watermelon mosaic virus 2(WMV)
WMV 2(WMV)
Zucchini yellow mosaic ZYMV
virus
Cucumber mosaic virus CMV
Unnamed potyvirus none

Papaya ringspot virus type W (PRSV-W),
previously known as watermelon mosaic virus
one (WMV 1), and watermelon mosaic virus
two (WMV 2) have been the most commonly
occurring aphid-transmitted viruses in Florida
for the past several decades. Zucchini yellow
mosaic virus has occurred in Florida sporadi-
cally since the fall of 1981. Cucumber mosaic
virus has been less prevalent during the past
30 years when compared to earlier periods, but
it has recently increased somewhat in peppers
and tobacco in north central Florida. Little is
known at this time about the unnamed
potyvirus, except that it is distinct from the oth-
ers.


Circular







Transmission


The predominant method of natural trans-
mission of the viruses listed in this publication
in the field is by aphids (sometimes called plant
lice, Figure 1) in a non-persistent manner. This
means the viral particles (virions) are acquired
by the aphid on its stylet (tubular mouthpart
which is inserted into plant tissue for feeding
purposes) and are retained in association with
the stylet for "short" periods of time. Although
an aphid is likely to lose its ability to transmit
the virus within a few minutes or a few hours,
longer periods of retention can occur. For ex-
ample, two species of aphids, Aphis gossypii and
Myzus persicae, were able to transmit ZYMV at
levels of 12.5% and 5.6%, respectively, for 3
hours at 680 F (210 C) but not at all after 5 hours.
In contrast, at 460 F (80 C). A. gossypii could trans-
mit ZYMV at a 1% level for up to 20 hours and
M. persicae could transmit at a 1% level for up
to 30 hours. The actual retention time for sur-
vival of virions on stylets is likely to vary.

In watermelon, no difference in susceptibil-
ity to infection to WMV 2 existed in plants from
1 1/2 to 7 1/2 weeks old in an experimental
situation. However, symptoms develop at dif-
ferent rates depending upon the age of the plant
at the time of infection. Older plants have an
uneven distribution of virus and lower levels
of virus when compared to the younger plants.
However, it is common for plants much older
than 7 1/2 weeks in commercial fields to be-
come infected with WMV 2 or other viruses.
Low relative humidity (e.g. 40%) has reduced
transmission of ZYMV by the aphid species A.
gossypii but not by the aphid species M. persicae.
The retention time of non-persistent viruses is
typically further shortened as feeding periods
lengthen. Thus, feeding on non-host tissue af-
ter acquisition of the virus, and before the sus-
ceptible crop is reached and probed by the
aphid, is likely to deplete viral inoculum. Re-
tention time is affected by so many variables
that it is not advisable to provide generaliza-
tions for a specific situation.


Aphids are efficient vectors of viruses. First,
they are sometimes able to acquire viruses in
less than eight seconds of probing and trans-
mit them in less than four seconds. Probes of
15 to 60 seconds can enhance transmission, but
longer probes tend to result in lower levels of
transmission. However, aphids may feed for
hours at one site if the host is suitable. Second,
aphids reproduce rapidly, resulting in millions
of aphids in a short period of time. Thirdly, alate
(winged forms, Figure 1) aphids are capable of
movement from one location to another either
by flying or by passive movement with the
wind. Winged forms tend to develop in re-
sponse to crowding from other aphids or when
plants deteriorate. Fourth, aphids can feed on
a number of plants within a short period of time.
They have "personal" feeding preferences and
will move from plant to plant of the same or
different plant species until they find a satis-
factory feeding site. Fifth, numerous species are
capable of vectoring viral diseases. For ex-
ample, at least 60, 42, 25, and 12 species of
aphids are known to be capable of vectoring
CMV, WMV 2, PRSV-W, and ZYMV, respec-
tively, on a world-wide basis.

In Florida the number of aphid species
known to vector WMV 2, PRSV-W, and ZYMV,
are 16, 9, and 13, respectively. Most of the spe-
cies do not reproduce on watermelon, but they
may land and probe while searching for their
preferred host. It is noteworthy that of these
species only a few species may account for a
majority of the transmissions in any given situ-
ation.

Aphid transmission of viruses is affected by
an enormous number of variables, and the in-
teractions between variables. For example, the
efficiency of aphid transmission may or may
not be altered when the aphid is carrying more
than one virus. In one situation, maximum
aphid transmission of CMV into cantaloupe
was delayed by one week when aphids ac-
quired a combination of CMV and WMV 2







when compared to CMV alone. Another ex-
ample is the extensive variation that exists in
transmission efficiency between aphid species
and clones of aphids within species as they in-
teract with other variables. For example, the
amount of transmission of WMV 2 from 10 dif-
ferent cucurbit species to cucumber with the
aphid species Aphis gossypii varied between 8
and 80% in one test.

The percentage of naturally occurring, mi-
gratory, alate (winged) aphids that carry trans-
missible virions. has been measured to be as
low as 1.5% for ZYMV and WMV 2 in one year.
In another year, none of 15,122 aphids was
found to be carrying transmissible virions. In a
situation with Florida-produced watermelons,
no aphids with WMV 2 could be detected at
the earliest time of epidemic development.
However, when 4% of the plants in a commer-
cial field had symptoms, 99% of the aphids in
the species Aphis middletonii were found to carry
WMV 2.
Aphid populations and viral diseases com-
monly progress over time in a sigmoidal fash-
ion, as do many other diseases. Typically,
flights of large populations of aphids occur
during dry periods accompanied by moderate
temperature. Such a situation occurs on a regu-
lar basis in south Florida during the fall (Octo-
ber-November) and in south, central and north
central Florida during the spring (March-May).
Within one to two weeks after these aphid
flights, viral symptoms are likely to appear.
Nearby fields will vary considerably with re-
spect to intensities and times of influx of peak
aphid flights.

Gradients of viral diseases are commonly
encountered in fields, particularly during the
early aspects of an epidemic. For example,
higher levels of disease often occur near edges
of fields or near known sources of inoculum.
These gradients become less evident over time
as the plants throughout the field become in-
fected from secondary spread.


Primary inoculum of virus comes from weed
hosts, abandoned cucurbit fields, nearby cucur-
bit fields, or volunteer cucurbits. Viruses are
introduced into a crop field by flights of aphids
where often an extremely low number of aphids
contain transmissible virions. It is generally
believed that because of the "short" retention
times of virions on stylets, sources of virus are
likely to be near the field. However, the reten-
tion times of 20-30 hours in some situations
suggests that more distant sources of virus
might also be involved.

The apparent rapid increase in the incidence
of diseased crop plants that we commonly see
is the function of secondary spread of virus. The
percentage of aphids that have acquired virus
from infected crop plants increases as the inci-
dence of disease increases. To exemplify how
fast epidemics develop, several epidemics in
watermelon required only 20 days to progress
from zero to 100% incidence.

Leafminers have been shown to transmit
WMV-2 and PRSV-W in squash. While this type
of transmission can occur, it has not been con-
sidered a major method of infection in Florida
in cucurbits.

Mechanical movement of plant sap from one
plant to another is another method by which
these viruses are spread. This can happen any-
time people or equipment move within the field
and make contact with infected plants. Such has
happened with CMV where the act of harvest-
ing cucumbers increased the incidence (num-
ber of infected plants) of disease three-fold.
Also, for experimental work, rubbing plant sap
from an infected plant to a healthy plant is a
common way of transmitting WMV-2, ZYMV,
PRSV-W, and CMV (Figure 8). While we nor-
mally think that aphid-transmission is highly
important for the spread of these viruses, we
may be underestimating spread of viral dis-
eases by mechanical means-at least where
multiple harvests occur or where movement of
plant sap occurs in other ways.







Seed transmission has not been demon-
strated for PRSV-W or WMV 2. Seed transmis-
sion of CMV has been demonstrated in some
weed species, which could conceivably serve
as a mechanism to perpetuate the virus in weed
hosts. Also, strains of CMV were found to be
seedborne in cowpea and bean. Seed transmis-
sion has occurred with ZYMV at 0.048% in one
study but in other studies seed transmission
did not occur. Even though seed transmission
of ZYMV is infrequent and at low levels, it does
provide a mechanism for ZYMV or any of its
related strains or any other virus to be distrib-
uted to new locations on a world-wide basis.

A virus that is seed-transmitted in canta-
loupe and squash and that has occurred rarely
in Florida is squash mosaic virus (SMV). SMV
is also transmitted by cucumber beetles, but not
by aphids. We mention SMV here because
sometimes individuals talk about mosaic
symptoms in squash as "squash mosaic". The
mosaic viruses commonly seen in squash in
Florida are WMV 2, PRSV-W, ZYMV, and
CMV, not SMV.

Symptom Expression

Upon mechanical or aphid transmission of
viruses to susceptible plants, infection becomes
systemic throughout the plant. Expression of
symptoms can be modified when mixed infec-
tions of two or more viruses occur within the
same plant. Mixed infections with PRSV-W,
WMV 2, or ZYMV occur fairly often. Variations
in temperatures, light regimes, crop varieties,
viral strains, host vigor, titer (concentration) of
virions in host tissue, and many other factors
interact with each other in complex and often
unknown ways to cause variation in symptom
expression in plants. For example, with PRSV-
W, WMV 2, and CMV, symptoms occurred 3 to
7 days after inoculation at 1050 F (40.60 C), com-
pared to 8 to 11 days at 650 F (18.30 C). How-
ever, progressive diminution of symptoms oc-
curred with five weeks of exposure to tempera-
tures between 860 F (300 C) and 1050 F. At the


highest temperatures, symptoms were absent
or barely visible. At 650 F, symptoms main-
tained their original integrity.

The first symptoms can appear as early as
three to four days after inoculation or as late as
18 or more days. Typically, plants inoculated
with infectious sap manually or via aphid trans-
mission will display symptoms seven to 12
days later. The absence of symptoms is no guar-
antee that the plant is not infected. An asymp-
tomatic (without symptoms) plant may be in-
fected but the incubation period for the virus
may not have been completed or in some envi-
ronmental conditions, the symptoms are not
displayed (sometimes referred to as "masked").
Symptoms may be present in one portion of a
plant but not in another portion of the same
plant. Symptomatic plants are more likely to
serve as sources of inoculum.

One should never rely on symptoms alone
to identify specific viruses. Also, chemical dam-
age from certain pesticides (usually herbicides)
or nutrient imbalances can cause symptoms
similar to those caused by viruses. Table 2 lists
symptoms that are caused by PRSV-W, WMV
2, ZYMV, or CMV.

Variations of mosaic and mottle symptoms
in leaves are common and are exemplified in
Figures 2, 3, 4, 5, 7, 8, 9, 10, & 14. Shading the
leaves on sunny days usually aids in contrast-
ing the lighter and darker portions of mosaic
patterns in leaves (Figures 4 & 8). Vein band-
ing is also common and is shown in Figures 3
& 5. Leaf bronzing (Figures 4 & 5) has been com-
mon in recent years in watermelon. Necroses
of margins or interior portions of leaves also
occur (Figure 6). Crinkling (rugosity) of leaves
is fairly common (Figure 6). Linear or stippled
greening of fruit is very common (Figures 2, 7,
10, 11, 12, 14, 15, & 16). Bumpiness of fruit is
fairly common (Figures 12 & 13). Ringspotting
can occur on leaves or fruit (Figures 9, 10, 11, &
15). Chlorotic spotting is sometimes induced
in the foliage (Figure 8).










Papaya ringspot virus type W
(PRSV-W, WMV 1)


It is not known when PRSV-W first occurred
in Florida but it is likely to have been in Florida
during the early 1930's. Currently, PRSV-W
predominates in the southern half of the penin-
sula throughout the year. PRSV-W occurs in the
northern half of the state but its appearance
there is usually during the late spring, summer,
or fall months. It may occur early in the spring
in central or northern Florida if infected trans-
plants from south Florida are used. PRSV-W
has been found as far north as New York, but
overall, PRSV-W is not found as commonly in
other areas of the United States as is WMV 2.
PRSV-W tends to occur in tropical climates.


PRSV-W infects 40 plant species with 38 of
them being in the cucurbit family and two of
them in the goosefoot family (Chenopodiaceae).
Type W strains of PRSV do not infect papaya
whereas type P strains (PRSV-P) do infect pa-
paya. PRSV-W and PRSV-P are very closely
related serologically.



PRSV-W is commonly found in a perennial,
cucurbitaceous weed, balsam apple (Momordica
charantia, Figure 17) in south Florida. Balsam
apple is a vine that is an excellent carry-over
host for PRSV-W and is commonly infected with
this virus. This weed is cold-sensitive and does
not grow in north Florida. Thus, it is not a di-
rect source plant for PRSV-W in north Florida.
It is particularly common in Palm Beach
County. It is found growing along irrigation
canals, hedge rows, wind breaks, old cultivated
fields, disturbed land, groves, or fences.


Table 2. Some symptoms that are caused by
PSRV-W,WMV 2, ZYMV OR CMV.

Leaf Fruit Other
Fewer flowers
Mosaic Mosaic

Plant stunting
Mottling Green mottling

Flower
Interveinal Flower
chlorosis Bumps Clustering
chlorosis

Greenish
Leaf strapping Shape flowers
distortion

Shortened
Rugosity Smaller size Internodes
(crinkled)

Marginal Lower sugar Deformed seed
Lower sugar
necrosis
Slower plant
Interveinal
necroi Ringspots Growth
necrosis

Plant
Bronzing Green spots deterioration

Shorter vines
Reduced size Off color Shorter vines

Distorted
Curling Fewer seeds Stigmas

Distorted
Vein banding* External cracks Stamens

S Abnormal Flower necrosis
Deep serrations
Netting #
Variable flower
Malformation Lower weight Color


Ringspots Fewer fruit

Chlorotic
Chlpotic Greenspotting
spotting
Dark green Inclusion
spotting Bodies**
Inclusion
Bodies**
*Yellow or green; # for cantaloupe; ** Microscopic
within cells







Another weed host for PRSV-W is creeping
cucumber (Melothria pendula, Figure 18). This
weed grows as an annual or perennial in fence-
rows, ditchbanks, abandoned fields, woods,
and other non-crop sites. It occurs in north, cen-
tral and south Florida. Creeping cucumber is
commonly infected with PRSV-W in south
Florida and sometimes in central Florida. It is a
major weed host for PRSV-W and WMV 2 in
southwestern Florida. Infected creeping cucum-
ber along edges of watermelon fields has been
strongly associated with outbreaks of PRSV-W
in southwest Florida even before peak periods
of aphid flights. Although freezing weather can
kill the vines in south Florida, regeneration of
new vines allows the plant to be a continual
source of virus because of the systemic infec-
tion. Because this weed is cold sensitive and it
typically does not grow in the northern portions
until after the spring crop is harvested, it is not
likely to be a source of PRSV-W or any other
virus for the spring crop unless an extremely
mild winter occurs. It has not yet been identi-
fied as a source plant for virus infections in the
fall crop for north Florida.


Watermelon Mosaic Virus 2 (WMV 2)

WMV 2 was first distinguished from WMV I
(PRSV-W) in 1965. It is serologically distinct
from PRSV-W. As with most viruses, multiple
strains of WMV 2 exist. WMV 2 has been the
most common aphid-transmitted virus of cu-
curbit crops in north Florida, in the United
States, and in other places in the world. For a
couple of years in the early 1960s, WMV 2 was
found commonly even in south Florida.

WMV 2 has a much broader host range than
does PRSV-W. Over 160 dicotyledonous spe-
cies in 23 families are susceptible. Many le-
gumes are susceptible. In central Florida, WMV
2 has been found in ornamental hibiscus plants,
which are perennials and could serve as carry-
over hosts. Annual plants found to be infected
with WMV 2 in central Florida include citron,


showy crotalaria, hairy indigo, and one-leaf clo-
ver. Balsam apple and creeping cucumber (see
PRSV-W section about these two weeds), both
natural carry-over hosts for PRSV-W, have not
been found naturally infected with WMV 2 in
the field, even though both are susceptible. One
of the probable reasons that WMV 2 has not
been found in creeping cucumber or balsam
apple is that WMV 2 is not found commonly in
south Florida where creeping cucumber and
balsam apple abound. Also, the growth of
creeping cucumber in north and central Florida
follows the spring cucurbit crops.

Zucchini Yellow Mosaic Virus (ZYMV)

Zucchini yellow mosaic virus was first found
in northern Italy in 1973 and in Florida in the
fall of 1981. ZYMV has occurred throughout the
main cucurbit-growing areas of Florida and the
United States, including Hawaii. ZYMV has
occurred in Africa, the Middle East, Asia, Aus-
tralia, and South America. It can be a problem
in both tropical and temperate areas. During
the early 1980's, ZYMV appeared in numerous
places in the world in a short period of time
(see an above section about seed transmission
of ZYMV). Recently, ZYMV remains as an im-
portant virus, but it has not dominated the spec-
trum of cucurbit viruses in Florida as do WMV
2 and PRSV-W.

One of the initial concerns about ZYMV was
the intensity of the resulting symptoms. ZYMV
seems to be associated with severe symptoms,
i.e. excessive leaf deformation, intense spotting,
pronounced bumpiness of fruit, etc.

Although ZYMV has a large host range as
determined by experimental inoculations, it
seems to be a commercial problem only in cu-
curbits. It has occurred naturally in creeping
cucumber in central Florida. ZYMV has not
been found naturally in balsam apple nor has
this host been susceptible when inoculated with
isolates from Florida. However, balsam apple
has been infected by isolates in France.








Cucumber Mosaic Virus

Cucumber mosaic virus (CMV) has an ex-
tremely wide host range and is found world-
wide. About 800 monocotyledonous and di-
cotyledonous plant species in 40 families are
susceptible. Perennial ornamentals such as
gladiolus, periwinkle, easter lily, and amaryl-
lis are examples from the host range.

Like ZYMV, CMV is capable of causing ex-
tremely severe symptoms. CMV was identified
in numerous vegetables, ornamentals, and
weeds in south and central Florida from the
1930s to the 1950s, but it was particularly a prob-
lem in celery (then called southern celery mo-
saic virus on celery and CMV on other crops)
at that time. By the late 1950s and early 1960s,
progressively fewer reports about CMV were
available. From the mid- 1960s till the late 1980s,
CMV was present but of no economic conse-
quence to cucurbits, vegetables, or agronomic
crops in Florida.

CMV had a negative economic impact on the
gladiolus industry which was solved in part
with the continual renewal of plant materials
with tissue culture-generated plants that were
free of CMV. CMV is considered to be the cause
of the demise of the Easter lily business in
Florida

During the 1990s, the incidence of CMV has
increased considerably in squash, peppers, and
tobacco in the Alachua County area. Interest-
ingly, dayflower (Commelina communis, Figures
19 & 20) has been found commonly in Alachua
County during this time in lawns, gardens,
open woods, fields, and hedgerows, often in
wet or shaded sites. It can grow as an annual or
perennial. A different species of dayflower
(Commelina nudiflora) was found to be associated
strongly with epidemics of CMV in celery in
the 1930s. In recent years, C. benghalensis has
been infected with CMV in Hamilton County
and associated with severe epidemics of CMV


in tobacco. Over the past 30 or so years, CMV
has not been a problem in cucurbits in Florida,
including cucumber. Some consider CMV to
spread more slowly than the other viruses, but
if infected Commelina spp. are in the vicinity,
CMV can be devastating in a short period of
time. Strains of CMV vary in their ability to in-
fect watermelon, but for at least the past three
decades, CMV has not been a problem in wa-
termelon in Florida.

Control

Resistant varieties are the best controls for
viral diseases. Resistance to CMV became avail-
able in 1928. Since that time, breeding programs
have successfully incorporated resistance to
CMV, other viruses, and fungal diseases in
pickling and slicing types of cucumbers. This
is the primary reason why viral problems in
cucumber are not common in Florida.

Recently, useable and stable resistance to
viral diseases in varieties of yellow and zuc-
chini summer squashes has become available.
The sources of resistance have been of two
types, host resistance and pathogen-derived
transgenic resistance. Genes from breeding
lines of squash with host resistance to WMV 2,
ZYMV, and CMV have been incorporated into
horticulturally acceptable varieties (e.g. 'Divi-
dend'& 'Revenue', zucchini types). Near 100%
control of WMV 2 was measured in a test in
Florida. Additional varieties will be available
in the future.

With another form of host resistance, a mask-
ing of greening in fruit occurs in infected plants.
Thus viral symptoms occur in the leaves but
are delayed in the fruit. This form of resistance
has been noted as having the "precocious-yel-
low-character" (e.g. varieties 'Multipik',
'Supersett', 'and Meigs') and has been effective
for WMV 2. In the future, more varieties of this
type may be available.

Another source of resistance has been de-







rived from the viruses themselves. It has been
known for some time that introduction of coat
protein of some viruses into plants can confer
an "immunogenic"-like response in plants. By
incorporating segments of nucleic acid from the
virus that encode for the protein coat into "mes-
senger" bacteria which in turn deliver a new
form of the gene into plant cells, plants are pro-
duced, via -tissue culture, that carry these new
genes that confer resistance. This new type of
resistance is referred to as pathogen-derived,
"transgenic resistance" (e.g. as developed in the
yellow summer squash varieties 'Prelude II',
'Liberator III', and 'Destiny III'). In a test in
Florida 100% control of WMV 2 was obtained
with all of these varieties. Additional varieties
are likely to be available in the future.

Although these forms of resistance have been
successful in Florida, one should remember that
the incorporation of resistance is subject to
"overthrow" by new strains of the virus for
which resistance was not incorporated origi-
nally. Also, resistance to one virus does not
typically confer resistance to other viruses. In-
terestingly, some coat protein-mediated resis-
tance to one virus has conferred some resistance
to other viruses. A lot of uncertainty still exists
about transgenic resistance.

With the current resistance to WMV 2,
ZYMV, and CMV in summer squash, geo-
graphical areas that commonly have these vi-
ruses will have some control. Such is the case
in north Florida in the spring. However, in the
fall in north Florida, when PRSV-W is likely to
be present, the resistance to the three other vi-
ruses will not control PRSV-W. Likewise, in
south Florida where PRSV-W abounds all year,
the varieties with resistance to WMV 2, ZYMV,
and CMV will not suppress PRSV-W. Resis-
tance to PRSV-W in squash is being pursued.

Some resistance to WMV 2 and ZYMV has
been found in breeding lines of watermelon.
At the present time, these lines lack horticul-
tural acceptance, but work is being done to de-


velop varieties of watermelon with resistance
to WMV 2.

Plow down abandoned fields as soon as
harvesting is complete. This will eliminate po-
tential sources of inocula for viruses and other
pathogens.

Destroy weed hosts (primarily creeping cu-
cumber and balsam apple) in the near vicinity
of the field. Weed hosts can be major sources
of inocula for aphid transmission.

Destroy volunteer cucurbits in the vicinity
of your production fields or on the farm as they
can serve as reservoirs for viral inocula.

Windbreaks have been used with partial
success to reduce spread of viral diseases from
one site to another. Windbreaks are particularly
useful where sequential plantings occur. When
aphids move from their original site they may
be "trapped" by the windbreak plants where
their feeding might deplete the virus from their
stylets before the aphids move to the suscep-
tible crop. This technique has been used suc-
cessfully in southeastern Florida where spaced
windbreaks of tall grasses, sugarcane, palm
trees, melaleuca (naturally occurring), etc. run-
ning north and south trap some aphids that
migrate in conjunction with predominate east-
erly winds. It would be best not to use
melaleuca as a windbreak because it is a good
host for the aphid, Aphis gossypii.

Roguing infected plants at first occurrence
of disease can be attempted, but generally this
technique does not keep pace with an ongoing
epidemic. Also, the risk of spreading disease
by shaking off aphids or by mechanical spread
exists.

Insecticidal sprays are not effective for the
viruses mentioned herein. By the time the aphid
has received a lethal dose, the virus has been
transmitted. Most studies have demonstrated
either no control, extremely slight control, or







increased disease. The latter occurs when the
aphids are agitated by the spray and thus move
to a new site where they inoculate other plants
before dying.

Oil sprays have been known for some time
to reduce progress of aphid-transmitted dis-
eases. Currently, the best such oil is one known
as 'JMS Stylet Oil'. It is effective if multiple
sprays are applied at a spray pressure of 400
psi with the specific nozzles indicated on the
label. Reducing primary infections by starting
the spray program early will be beneficial
whereas waiting until the incidence of disease
becomes 10% or more will not provide control.
Whenever oils are used, risk of phytotoxicity
exists from the oil or its interaction with other
sprayed materials.

Row covers have been used with partial suc-
cess in reducing aphid-transmitted viruses in
cucurbits. They do so by being a mechanical
barrier and by the repellency of aphids with the
light and somewhat shiny color of the row
cover. However, as soon as the covers are re-
moved for accommodating bee visitation for
pollination, one can expect infection to occur
with symptoms beginning in 1 to 2 weeks. In-
terestingly, row covers left on zucchini squash
for up to 10-12 days after initial flowering re-
sulted in higher yields compared to those with-
out row covers because delaying disease over-
rode the benefits of an earlier removal of the
covers for pollination. The benefit from row
covers is that it delays the initial onset of dis-
ease which could at least increase yields of the
early pickings. The use of closed systems that
exclude aphids but allow for activity of bees
would be an excellent control method for these
viral diseases.

Reflective mulches that have some bright-
ness and shine are repellent to aphids and can
delay the onset of viral diseases. Usually silver
or aluminum mulches will be more repellent
to aphids than white mulches. Once the mulch


is covered by plant growth, the effect is lost,
but the delay in overall disease progress may
be worthwhile. Reduction of disease incidence
of WMV 2 has been as high as 97% at a given
time and averaged 63% in a test on squash in
California with aluminum mulch. In tests in
Alabama, delays of 10 to 13 days for onset of
viral epidemics from PRSV-W, WMV 2, ZYMV,
and CMV occurred with aluminum mulch. In
Florida, reductions of viral disease by 72 and
94% have been measured with use of alumi-
num foil. In another test in Florida, no signifi-
cant benefit was measured with white on black
plastic. Combining the use of row covers with
either white or aluminum mulches has signifi-
cantly reduced viral diseases in cucurbits in
Florida and Oklahoma.

Protection (trap) crops, where a non-host
plant is inter-planted with the cucurbit crop to
trap aphids, have reduced viral diseases. This
technique could be useful in specialty plantings
such as in gardens or "organic" production
sites. It would be highly labor intensive and not
compatible with logistical operations and
equipment.

Time of planting or transplanting can influ-
ence levels of disease. In north Florida, earlier
planting or transplanting will allow the plant
to be at a further stage of development before
aphid flights begin. At least the earlier pickings
might escape some damage. While transplant-
ing rather than planting seed does not always
increase yield, transplanted crops tend to have
earlier yields.

Healthy transplants should always be used.
Failure to use disease-free transplants com-
monly results in significant loss in yield and
quality. Avoid purchasing plants from places
that have had a history of producing diseased
transplants.

Disease-free seed should always be used.
Unfortunately, the grower has little recourse on
this matter except to ask questions of represen-







tatives from the seed companies. Except for
SMV and ZYMV, seed transmission of mosaic
viruses in cucurbits has been of little conse-


quence in Florida.
Integration of the above tactics should be
done to the extent possible. Integrated disease
control systems should


Figure 1. Winged aphid. Figure 2. Mosaic in squash leaf and fruit
(ZYMV).


. I


r


U';
'V.

'-I\


Figure 3. Green vein-banding in watermelon Figure 4. Leaf mottling and bronzing in
leaf (WMV-2). watermelon leaf (Note: enhancement of
symptoms with shading).

















I 1


Figure 5. Bronzing and dark and light vein
banding in Watermelon leaves (WMV 2).


Figure 7. Vein banding in leaf and mottling
in butternut squash leaves and fruit
(ZYMV).


Figure 6. Necrosis and rugosity in pumpkin
leaf (WMV 2 and PRSV-W).


Figure 8. CMV-inoculated resistant and
susceptible squash.



























Figure 9. Ringspotting in immature pump-
kin fruit (WMV 2 and PRSV-W).


Figure 11. Mosaic in watermelon fruit
(WMV 2).


Figure 10.Ringspotting in mature pumpkin
fruit (WMV 2 and PRSV-W).


Figure 12. Mosaic and bumps in watermelon
fruit (ZYMV + WMV 2).


j 7'
* Sif .2 .



























Figure 13. Greening and bumps in yellow
summer squash fruit (ZYMV).


Figure 15. Mottling in acorn squash fruit.


Figure 14. Mottling in yellow squash fruit
(ZYMV).


Figure 16. Mottling in yellow summer
squash fruit.



























Figure 17. Balsam Apple. Figure 18. Creeping cucumber.


Figre 20. Dayflower with flowers.


Figure 19. Small dayflower.




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