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 Front Cover
 Table of Contents
 Introduction
 Materials and methods
 Methods of application
 Fungicide evaluation
 Seasonal occurance of the...
 Summary
 Literature cited














Group Title: Bulletin - University of Florida Agricultural Experiment Station ; 596
Title: Factors affecting the incidence and control of northern corn leaf blight in sweet corn production in the Everglades
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 Material Information
Title: Factors affecting the incidence and control of northern corn leaf blight in sweet corn production in the Everglades
Series Title: Bulletin - University of Florida Agricultural Experiment Station ; 596
Physical Description: Book
Language: English
Creator: Cox, Robert S.
Harrison, D. S.
Publisher: University of Florida Agricultural Experiment Station
Publication Date: 1958
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Bibliographic ID: UF00027647
Volume ID: VID00001
Source Institution: University of Florida
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Table of Contents
    Front Cover
        Page 1
    Table of Contents
        Page 2
    Introduction
        Page 3
        Page 4
    Materials and methods
        Page 5
    Methods of application
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
    Fungicide evaluation
        Page 10
        Page 11
        Page 12
        Page 13
    Seasonal occurance of the disease
        Page 14
        Page 15
        Page 16
        Page 17
        Page 18
    Summary
        Page 19
    Literature cited
        Page 20
Full Text



Bulletin 596


May 1958


UNIVERSITY OF FLORIDA
AGRICULTURAL EXPERIMENT STATIONS
J. R. BECKENBACH, Director
GAINESVILLE, FLORIDA
(A Contribution from the Everglades Experiment Station)





Factors Affecting the Incidence and Control of Northern
Corn Leaf Blight in Sweet Corn Production
In the Everglades

R. S. Cox and D. S. HARRISON



TECHNICAL BULLETIN


Fig. 1.-Effect of nozzle arrangement on disease control. This plot was
sprayed with two nozzles overhead and one to three on each side. Note
heavy foliage and number and size of ears. Compare with Figures 2 and 3.


,,1'








CONTENTS
Page
INTRODUCTION ................... .............. 3
MATERIALS AND METHODS .... ...... .--.----....------ .....-- ----- ...- ----.. 5
M ETHODS OF APPLICATION .... .... ................. ... ... .... 5
N ozzle Type ................................ ..... ... .. ...... .... .. ... 5
N ozzle Placem ent .... ........ ... .... ..... -... .. .. ... .... 5
G allonag e ..... .... ------ ..... .. ......... ..... .... .- ... .. -... 8
P pressure ............ ... ...... ...... .... .. .. .. 8
FUNGICIDE EVALUATION ....... ............ .. .... ...... .... .. ...... .. 10
D disease Control ....... ....... ....... .... ... ... 10
Phytotoxicity ... ........ ....... .. ... ..... .... .... .... .. .. .. ........ 13
SEASONAL OCCURRENCE OF THE DISEASE ....... ..................... ...- ..... 14
SUM MARY 1........ .... ............ 19
LITERATURE CITED ... ............... ...... ... ... ..... ..... .. ... .. ... ... 20




RECOMMENDATIONS FOR CONTROL

1.) Materials to use (amount in 100 gallons of water):
a. First choice: Maneb-Dithane M-22 or Manzate (11/2 lbs.),
or Parzate (2 lbs.) plus Thylate (11/. lbs.).
b. Second choice. Parzate (2 lbs.).
c. Third choice: Nabam plus zinc sulfate (2 qts.-%, lb.),
Amobam plus zinc sulfate (1 qt.-%l lb.) or Dithane Z-78
(2 lbs.).

2.) Nozzle placement: Two nozzles over the row. One to three
drop nozzles, (determined by height of corn) spaced on 15-
inch centers.

3.) Gallonage: Variable, depending on size of corn and disease
severity. As low as 60 gallons per acre are adequate on
small corn, and as much as 250 gallons per acre may be nec-
essary for best protection on corn in the tassel stage under
epiphytotic conditions.
4.) Pressure: 200-400 p.s.i.

5.) When to start spraying: When disease is first observed in
planting, but not before two weeks after seeding.

6.) How often to spray: Twice weekly if weather conditions are
favorable for disease. Otherwise, once weekly. The criti-
cal period for infection, thus the critical period for spray-
ing, is during the time the corn is in the whorl stage, i.e.,
the pre-tasseling period.








Factors Affecting the Incidence and Control of Northern
Corn Leaf Blight in Sweet Corn Production
In the Everglades

R. S. Cox and D. S. HARRISON

During the past 10 years the sweet corn industry of south
Florida, particularly the Everglades area, has emerged from a
status of relatively minor importance to a major role in the agri-
cultural economy of the area. Total acreage for the 1947-48
season was estimated at 6,000. By 1955-56 the acreage, tripling,
was estimated at 18,000 (6). Intensification of a vegetable crop
in south Florida generally is followed by the development of im-
portant disease problems. Sweet corn is no exception. It became
evident that maximum production was contingent upon control
of the Helminthosporium leaf blight diseases.
In 1954 Stoner et al. (8) reported that nabam or zineb gave
effective control. Although most of the growers experienced
reasonably good yields during the following three-year period,
1953-54-55-56, some obtained poor yields each year through
failure to control disease. This probably was due more to im-
proper methods of application than to use of the wrong materials.
Even where good control was obtained, the methods of applica-
tion employed were not the most efficient. Because of this, it
was speculated that even the best growers could suffer consider-
able losses during severe seasons. This observation was brought
sharply into focus during the spring of 1957.
As a result of an understandable apathy resulting from sev-
eral previously mild seasons, many growers failed to take heed
of the danger signals which became evident in January of 1957.
The first three months of that year proved to be the wettest in
history. Nevertheless, the growers continued to spray their corn
as usual. The results were disastrous. Several thousand acres
were abandoned, and yield and quality from much of the re-
maining were greatly impaired. Losses ran into millions of
dollars. It is hoped that the information presented in this bul-
letin on seasonal occurrences and on control of northern corn
leaf blight (Helminthosporium turcicum Pass.) may help obviate
the recurrence of the 1957 disaster.
Cox: Formerly Associate Plant Pathologist; Harrison, Assistant Agri-
cultural Engineer, Everglades Experiment Station. The writers are in-
debted to Nelson Nelms, formerly field assistant, for supervision of the field
work, to Henry M. Spelman III, Staff assistant, for assistance in photog-
raphy, and to E. King, Jr., draftsman, for preparation of the graphs.





























Fig. 2.-Effect of nozzle arrangement on disease control. This is the
unsprayed check (note defoliation and absence of ear). Compare with
Figures 1 and 3.




Fig. 3.-Effect of nozzle arrangement on disease control. This plot
was sprayed with one nozzle overhead and one to two on each side. Com-
pare with Figures 1 and 2.





S ., .-.. ,. .
.^ S:.* -\ ***






Northern Corn Leaf Blight in Sweet Corn'


MATERIALS AND METHODS

The relative effectiveness of various fungicides against north-
ern corn leaf blight on Golden Security corn was compared in
field experiments during the spring seasons of 1955-56-57. Plots
consisted of single 50-foot rows spaced three feet apart. Each
plot was bordered on either side by a single guard row. Experi-
mental design consisted of randomized blocks with four replica-
tions. Materials were applied with the spray rig described else-
where (3).
For the tests involving methods of application, plots consisted
of three 50-foot rows with a three-row guard on either side. A
split plot design was employed. Materials were applied with a
six-row sprayer. Nabam plus zinc sulfate (2 qts.-3/ lb. 100 gal.)
was used throughout.
Unless otherwise indicated, the Myer's Jumbo nozzle with a
No. 3 disc was used in all tests and a pressure of 300 p.s.i. was
maintained. Standard cultural procedures and recommended in-
secticide programs were followed.
Fungicide spray operations were started two to five weeks
after seeding, and applications were made twice weekly. The
number of applications ranged from 12 to 16. Plots were scored
for disease severity following the scheme of Horsfall and Barratt
(7). Yield data relative to number and weight of marketable
ears were obtained.

METHODS OF APPLICATION
The efficiency of various nozzles as determined by disease
control was studied in 1955; the effect of nozzle placement and
gallonage per acre, in 1956 and 1957; and the effect of pressure,
in 1957.
Nozzle Type.-This test was run in 1955 under conditions of
mild to moderate disease (Fig. 6). Under these conditions, the
Myer's Jumbo nozzles with No. 3 disc, Myer's Fembro nozzle
with No. 3 disc and the TeeJet T10 performed equally well, as
determined by disease severity and yield. This suggested that
factors such as original cost and durability, rather than per-
formance, should be used in selection of nozzle type. Complete
results have been reported in Florida State Horticultural Society
Proceedings in 1955, Vol. 68, pp. 213-216.
Nozzle Placement.-This test was run under relatively mild
conditions for the Everglades area in 1956, and repeated in 1957






Northern Corn Leaf Blight in Sweet Corn'


MATERIALS AND METHODS

The relative effectiveness of various fungicides against north-
ern corn leaf blight on Golden Security corn was compared in
field experiments during the spring seasons of 1955-56-57. Plots
consisted of single 50-foot rows spaced three feet apart. Each
plot was bordered on either side by a single guard row. Experi-
mental design consisted of randomized blocks with four replica-
tions. Materials were applied with the spray rig described else-
where (3).
For the tests involving methods of application, plots consisted
of three 50-foot rows with a three-row guard on either side. A
split plot design was employed. Materials were applied with a
six-row sprayer. Nabam plus zinc sulfate (2 qts.-3/ lb. 100 gal.)
was used throughout.
Unless otherwise indicated, the Myer's Jumbo nozzle with a
No. 3 disc was used in all tests and a pressure of 300 p.s.i. was
maintained. Standard cultural procedures and recommended in-
secticide programs were followed.
Fungicide spray operations were started two to five weeks
after seeding, and applications were made twice weekly. The
number of applications ranged from 12 to 16. Plots were scored
for disease severity following the scheme of Horsfall and Barratt
(7). Yield data relative to number and weight of marketable
ears were obtained.

METHODS OF APPLICATION
The efficiency of various nozzles as determined by disease
control was studied in 1955; the effect of nozzle placement and
gallonage per acre, in 1956 and 1957; and the effect of pressure,
in 1957.
Nozzle Type.-This test was run in 1955 under conditions of
mild to moderate disease (Fig. 6). Under these conditions, the
Myer's Jumbo nozzles with No. 3 disc, Myer's Fembro nozzle
with No. 3 disc and the TeeJet T10 performed equally well, as
determined by disease severity and yield. This suggested that
factors such as original cost and durability, rather than per-
formance, should be used in selection of nozzle type. Complete
results have been reported in Florida State Horticultural Society
Proceedings in 1955, Vol. 68, pp. 213-216.
Nozzle Placement.-This test was run under relatively mild
conditions for the Everglades area in 1956, and repeated in 1957







during one of the worst epiphytotics for northern corn leaf blight
in the history of the area. Pertinent data relative to nozzle ar-
rangement are listed in Table 1 and illustrated in Fig. 4.


<-------36"' -------->-3


IS
10A





18"





L- =


Florida Agricultural Experiment Stations


B









D


Fig. 4.-Overhead and side nozzle arrangements used in experimental
work. The nozzle arrangement corresponds to treatments listed in Table 1
as follows: A, treatments 2 and 3; B, treatments 4 and 5; C, treatments
6 and 7; D, treatments 8 and 9; E, treatments 10 and 11; and F, treatments
12 and 13.







Northern Corn Leaf Blight in Sweet Corn


TABLE 1.-NOZZLE PLACEMENT, SPEED OF RIG AND GALLONAGE
PER ACRE OUTPUT.

Treatment Nozzle Placement:__ Speed- Gal. per
Overhead-Each Side (m.p.h.) acre

1 (check) -
2 1 1 2.4 100

3 1 1 3.4 60
4 2 1 2.4 130
5 2 1 3.4 80
6 1 1 to 2:: 2.4 100-165
7 1 1 to 2* 3.4 60-100
8 2 1 to 2* 2.4 130-200
9 2 1 to 2* 3.4 80-120
10 1 1 to 3:' 2.4 100-230
11 1 1 to 3* 3.4 60-140
12 2 1 to 3* 2.4 130-260
13 2 1 to 3* 3.4 80-160

Depending on height of corn.
** Gallons per acre output depends on the number of nozzles used. For example, with
treatment 13, 80 gals./A were applied on the young corn when the nozzle arrangement con-
sisted of one nozzle on each side, 120 gals. with two drop nozzles, and 160 gals. with the final
arrangement on the mature corn of three drop nozzles (see Fig, 4).

In 1956, each treatment, i.e., nozzle arrangement, provided a
significant degree of control over that of the check. It was evi-
dent also that two nozzles placed over the row provided better
control than one, and that degree of control increased as the num-
ber of drop nozzles was increased from one to three (Figs. 1, 2
and 3).
These data were confirmed the following year (Table 2). Two
overhead nozzles were superior to one, from the standpoints of
both disease control and yield. The advantage of two or three
side nozzles over one was again suggested, although differences
were not nearly as great as in the case of the overhead arrange-
ment.
These findings clearly point out the importance of the over-
head nozzle arrangement. They also suggest that under mild
conditions one or two drop nozzles on either side of the row might
be sufficient. However, it is possible that the third nozzle,







Florida Agricultural Experiment Stations


though not affecting yield, might be wise insurance against blight
lesions on the husks. Such defect can throw the ears out of
grade or actually render them unmarketable.
Gallonage.-The effect of gallonage output was studied simul-
taneously with nozzle arrangement (Tables 1, 2). Each nozzle
arrangement was tested at a high and low gallonage output.
This was accomplished by driving the spray rig at different
speeds. In 1956 there was an indication that control was pro-
portional to the gallonage.

TABLE 2.-EFFECT OF GALLONAGE OUTPUT AND NOZZLE ARRANGEMENT ON
NORTHERN CORN LEAF BLIGHT DEVELOPMENT AND ON YIELD. EVER-
GLADES EXPERIMENT STATION, 1957.

Yield
I Disease Severity ) (marketable ears)
Treatments I weight
4/16 4/23 4/30 5/7 I5/14 number (lbs.)
Gallonage* Low 3.2 4.0 4.8 4.9 6.7 89 30.4
High 2.9 3.3 4.1 3.9 5.5 117 41.2


Overhead 1 3.1 1 3.9 4.8 4.8 6.6 88 30.1
Nozzles 2 2.9 3.4 4.1 4.0 5.6 118 40.1
1 5* *


Side 1 3.0 3.7 4.7 4.6 6.3 99 34.6
Nozzles 2 3.0 3.5 4.3 4.3 6.2 103 35.6
3 2.9 3.6 4.4 4.2 5.8 106 37.1
N.S. N.S. N.S. N.S. N.S.

Readings for low gallonage were derived by averaging all those from plots sprayed with
the rig driven at the high speed. Likewise, high gallonage readings were obtained from
readings from plots sprayed with the rig driven at the low speed (see Table 1).
** Significant differences at the 1 percent level.
N.S. No significant differences.

The value of high gallonage under severe conditions was
clearly demonstrated in 1957 (Table 2). With all nozzle ar-
rangements employed, the disease was less and yield higher
where the higher gallonage output was used (Figs. 1, 2 and 3).
Pressure.-A test was run in 1957 to gain information on
the effect of pressure on disease control where gallonage per acre
was held constant. Predetermined ground speeds of the spray
rig were employed to insure equal gallonage outputs with each
of three test pressures (200, 300 and 400 p.s.i.). No significant
differences were noted between the test pressures as reflected in
disease control or yield, although there was some indication that








Northern Corn Leaf Blight in Sweet Corn 9


TABLE 3.-EFFECT OF SEVERAL FUNGICIDES ALONE AND IN COMBINATION ON
THE CONTROL OF NORTHERN CORN LEAF BLIGHT ON SWEET CORN. EVER-
GLADES EXPERIMENT STATION. 1955.


Materials Concentration Disease
in 100 gals. Severity


None (check) .
Dithane M-22 .....
M anzate ...............
Dithane D-14
36 percent
zinc sulfate ......
Dithane Z-78 ......
Zerlate ................
Vancide Z-65 .......
C & C 7443 ........
Fermate ..............
Z-C Spray ...........
Vancide MX 427....
Orthocide 50-W ....
Agri-mycin ........
XSF.3 .......... ...-
Vancide F-1655W

L.S.D. 0.05 ..........
L.S.D. 0.01 ...........

Significant increase
** Significant increase


Yield (lb.)
First
Total (upper)
ear


11' lb.
312 lb.

2 qts. --
3i lb.
2 lb.
2 lb.
2 lb.
2 lb.
2 lb.
2 lb.
2 lb.
3 Ib.
200 ppm.
3 pts.
2 lb.


6i N.S.
8 N.S.


over the check at the 5 percent level.
over the check at the 1 percent level.


1 Active ingredients and percentage composition of materials not listed under footnote 1
Table 1: Manzate, 70 percent manganous ethylenebis (dithiocarbamate ; Z-C spray, 76 per-
cent zinc I.r., 1. i li. i ...r .. in 100, 15 percent streptomycin sulfate and 1.5
percent 1 I ... I .... i ... . i ,-, 1. -3.. .6,10,12,12a-hexahy-
droxy-(-methyl-l,11-dioxo-2-naphthacenecarhc ... I ., I 1 ', 75 percent ferric ni-
troso dimethyldithiocarbamate.
SScored May 19, 1955, on a 0-11 hasis where 0 indicates no disease and 11, complete
defoliation.
a Harvested May 24, 195;.


TABLE 4.-EFFECT OF MATERIALS AND CONCENTRATION ON DEVELOPMENT
OF NORTHERN CORN LEAF BLIGHT. EVERGLADES EXPERIMENT STATION, 1956.*


Materials


N one .................
M anzate .............. .
M anzate .............. .
Dithane M-22 ...... .......
Dithane M-22 .................
Parzate ........... .. .. .....
Parzate ..... .......... --...
Dithane Z-78 ................-.
Dithane Z-78 ..........-.........-.- .
Nabam plus zinc sulfate -..
Nabam plus manganese sulfate


Concentration
in 100 gals.


11 lb.
2 lbs.
1/2 lbs.
2 lbs.
2 lbs.
3 lbs.
2 lbs.
3 lbs.
2 qts. % lb.
2 qts. -1 lb.


Disease Severity
5 21/56


4.8
1.4
1.1
1.5
1.1
1.8
2.3
2.4
2.4
1.4
1.1


Yield data were not obtained because of poor stands.


Second
(lower)
ear

9
20*:



21
17:
19**
17*

15*
16*
16*
15:
11
9
12

6
9






Florida Agricultural Experiment Stations


a pressure of 300 p.s.i. was slightly more effective. The practical
implication of these findings is that where a shortage of equip-
ment exists, more acreage can be covered while at the same
time maintaining desired gallonage output by increasing pres-
sure appropriately with ground speed.

FUNGICIDE EVALUATION
During the course of this work a screening program was
maintained where the performance of standard as well as new
materials was compared. Data on disease control and phyto-
toxicity were obtained.
Disease Control.-A report on the performance of several ma-
terials against southern blight (H. maydis Nisikado and Miyake)
and northern blight during the 1954-55 season appears elsewhere
(2), and the results are reproduced in Table 3. The dithiocarba-
mate fungicides (nabam plus zinc sulfate, zineb, maneb) were
superior to others tested. In subsequent work during the 1955-
56 season, none of several antibiotic materials tested was appre-
ciably effective fungicidally. These included streptomycin sul-
fate, Anisomycin, kojic acid derivatives, and Actidione. Com-
parative performances of the dithiocarbamate fungicides are
shown in Table 4. Results of work in 1957 appear in Table 5.
Several points of interest are noted: a) maneb (Manzate, Dithane
M-22) showed superiority over zineb (Dithane Z-78, Parzate);
b) Parzate powder (zineb) showed better control than Dithane
Z-78 or Parzate liquid (nabam) plus zinc sulfate, especially dur-
ing the latter part of the season; c) nabam plus zinc sulfate
showed better control than nabam plus manganese sulfate; d)
the new ammoniacal carbamate, Amobam, plus zinc sulfate pro-
vided control equal to that of nabam plus zinc sulfate or Dithane
Z-78; e) the new material, Dyrene, was equal to nabam plus
zinc sulfate; f) Thylate (thiram) was relatively ineffective when
used alone; g) Parzate at 1 lb./100 plus Thylate at 1 lb./100 was
about as effective as Parzate at 2 lbs./100 alone; and Parzate at
2 lbs./100 plus Thylate at 11/ lbs./100 mixture was significantly
better than Parzate alone; h) even the relatively ineffective ma-
terial, Omadine, provided highly significant control as compared
to the check; i) ear quality was affected by the severity of
blight. Plots sprayed with the more effective materials produced
heavier ears (Table 5) better filled toward the tips (Fig. 5).
A compilation of data obtained from 1955 through 1957
shows some interesting comparisons concerning the perform-








TBI3LE 5.-EFFECT OF SEVERAL FUNGICIDES ALONE AND IN COMBINATION ON THE CONTROL OF NORTHERN
CORN LEAF BLIGHT ON SWEET CORN. EVERGLADES EXPERIMENT STATION, 1957.


Materials '


Concentration
in 100 Gals.


Check ... .. ........................
Dithane Z-78 ............ .. 2 bs.
Parzate ...... .................... .. 2 bs.
M anzate ................... 1'A lbs.
Dithane M-22 .. .....-..... ... ...- 1/ lbs. =
Parzate (nabam) + zinc sulfate ......... 2 qts.-% lbs.
Parzate (nabam) + manganese sulfate 2 qts.-1 lb.
Amobam + zinc sulfate .-................... 1 qt.-% lbs.
Amtbam .. ......................... 2 qts.
Dyrene .. .................... .. 2 lbs.
Thylate .. ......... lbs.
Parzate + Thylate ...................... 1 lb.-1 lb.
Parzate + Thylate ......... 1 lb.-11/ lbs.
Omadine .. ......................... .. 2 lbs.


Disease Severity

15 4/23 4/29 5/7


4.0
3,0
2.8
2.0
2.0
3.0
2.5
2.8
3.5
3.0
3.0
2.5
2.5
3.3


0.5 0.( 0.6
0.7 0.8 0.9


0.0 1 9.5
0.8 1 12.7


4.0 0.03
5.4 0.04


Active ingrt edients and percentage competition of materials listed as Follsows: Dithane Z-7S. 6(5 percent zinhb: Plrziatel, (i pelrent zoineb; Manzatc.
70 percent manerlb Dithane M-22, 70 percent maneb; liquid Parzate, 19 percent naham ;l Amobam, :' percent liammllnium ethylene Llisdithioniarbamate;
Amtbam, 20 percent ammonium tetraethylenelmine hisdithiocarbamate. Dyrtene. 50 percent 2,1 dichl(tr fi-I0-chl),ronnilino) triazine. T'hylatc, (i.6 percent
thiram; Omadine 50 percent Zn salt of 2-pyridinethione 1- oxide.
** Data include 1st and 2nd ear.
SSecond ear not included in these data.


5/14 Total
No. *


Yield (5/21/
Total
Weight *s
(lbs.)


57)
Avg. wt.
1st ear i
(lbs.)


L.S.D. .05
L.S.D. .01





Florida Agricultural Experiment Stations


?^-. **


I'


-rr 911


Fig. 5.-Effect of fungicides on ear fill: Ears from plots sprayed with
Thylate (upper left), Parzate (upper right), Parzate plus Thylate (lower
left), and Manzate (lower right).







Northern Corn Leaf Blight in Sweet Corn


ance of the various carbamate formulations (Table 6). The
superiority of maneb is evident. It also appears that the Parzate
formulation of zineb is more effective than the Dithane Z-78
formulation. Nabam (plus metallic salts) was quite effective,
except for the 1957 season.

TABLE 6.-COMPARATIVE EFFECTIVENESS OF ZINEB, MANEB AND NABAM
FORMULATIONS ON THE CONTROL OF NORTHERN CORN LEAF BLIGHT.

Materials Disease Severity (final rating)
1955 1956 1957

Maneb
M anzate ................................ 1.0 1.4 3.0
Dithane M-22 .... ....... .... 1.1 1.5 2.6
Average ........................... 1.05 1.45 2.8

Zineb
Parzate ............................... .. 1.8 3.5
D ithane Z-78 ....... .......................... 2 0 2.4 4.3
Average ....................................... 2.0 2.1 3.9

Nabam + Zn Sulfate ..... .......... 1.3 1.4 4.6
Nabam + Mn Sulfate ................ -- 1.1 5.8

Check ........................... ... ...... 5.0 4.8 9.0

All materials applied at standard rates in 100 gals., i.e., zineb formulations 2 lbs.;
maneb formulations, 11. lbs. nabam, 2 qtb.: zinc sulfate, 3/ lb.; manganese sulfate, 1 lb.

It is suggested that evidence presented here lends weight to
the proposition that workers should not report categorically re-
sults of fungicide evaluations under generic names. It is true
that both Parzate and Dithane Z-78 contain zineb as the active
ingredient. However, it would appear that differences occur be-
tween the formulated products. One needs only to examine the
residue on corn plants (or others) sprayed with each material
for substantiation.
Phytotoxicity.-Corn appears to be a very sensitive test plant
from the standpoint of phytotoxicity. The neutral coppers tested
caused severe injury (2). All of several antibiotics caused chlo-
rosis. These included streptomycin sulfate, kojic acid deriva-
tives, Anisomycin and Actidione. Of these, three separate formu-
lations of Actidione (25 ppm) caused most severe injury. The
severe chlorosis was followed by such acute stunting and lodging
of the plants that no ears were ever formed.






Florida Agricultural Experiment Stations


In 1953 Conover (1) reported injury with maneb at Home-
stead. None was observed at the Everglades Station during 1954
or 1955 (relatively dry seasons). During wet periods of 1956
and 1957, however, this material caused a contact leaf burn. The
injury occurred only when maneb was applied in the leaf whorl
of plants during rainy weather when free moisture persisted
for prolonged periods. It is believed that injury under such con-
ditions is the result of the soluble portion of maneb coming in
direct contact with succulent host tissues for extended periods.
In no case was this injury of sufficient magnitude to reduce yield.
Slight contact burn was noted with nabam plus zinc sulfate
for the first time during the spring of 1957, apparently because
of the wetness of the season. With the manganese salt, nabam
caused more severe injury in the form of a light to moderate leaf
burn.
Neither zineb nor thiram caused injury when used separately.
However, a light to moderate burn occurred when they were
applied in mixture to corn in the whorl stage. This suggests
that action of these materials is increased in some manner when
they are used together.

SEASONAL OCCURRENCE OF THE DISEASE
Correlations were studied between disease development and
weather, and between disease severity and yield during the spring
seasons of 1955, 1956 and 1957. An examination of the official
weather data obtained at the Everglades Experiment Station
revealed the following: 1) except on two occasions, maximum
relative humidity reached 100 percent every night during the
three seasons under study; 2) the average minimum relative
humidity for the first two (1955 and 1956) was 49 percent, for
the third (1957) it was 59 percent; 3) the mean maximum tem-
peratures for the three seasons were, respectively, 84, 82 and
83 F., the mean minimum temperatures, 57, 57 and 62 F.; 4)
total rainfall for each of the three seasons was 6.5, 3.4 and 12.8
inches, respectively. These data, together with disease indexes,
are recorded in Figures 6 to 8. These observations suggested
that the maximum relative humidities and maximum temper-
atures played only a minor role, at best, in the differential devel-
opment of the disease during the three seasons. They did show,
however, that minimum humidities, minimum temperatures and
total rainfall played important roles in disease development.






Northern Corn Leaf Blight in Sweet Corn


In 1955 the disease was present at a low level the first three
weeks after seeding. It increased gradually from the third
through the fifth week. During the latter part of the season it
increased through the ninth week sharply, then dropped off
slightly. The interpretation of the effect of weather on this
pattern is as follows:


I 2 3 4 5 6
Weeks
- Mar.9 1955


7 8 9 10


May 17 -


Fig. 6.-Average weekly minimum relative humidity and minimum tem-
perature, rainfall distribution and disease development for the 1955 spring
season.

Weather conditions must have been favorable for the disease
to become established from primary inoculum during the first
week after seeding. However, light rainfall, low relative humidi-
ty and low temperature held the disease in check during the first
three weeks. The three rains during the third and fourth weeks
and the subsequent rise in relative humidity and temperature
created conditions favorable for a disease cycle (s) into the fifth
week.
There was only one rainy period between the fourth and ninth
weeks. With a subsequent drying out of the soil, the average


--@ Disease Severity
S ---- Relative Humidity
S----- Temperature




/v\






Florida Agricultural Experiment Stations


minimum relative humidity decreased rather sharply. Thus, the
only weather factor favorable for disease development during
this period would appear to be temperature-the weekly mean
minimum fluctuated around 60 F. This, together with a con-
siderable inoculum potential and the residual effect of the rainy
sixth week, resulted in the build-up of disease toward the end
of the season. Had more rain occurred during this period the
disease probably would have reached epiphytotic proportions.
In the 1956 season the disease made its initial appearance
during the sixth week following seeding and increased slightly
during the eighth. It built up rather sharply from that time
until harvest. This is accounted for as follows:

70 -- Disease Severity
-o Relative Humidity
STemperature
S60 6


I 50 / i


40 2 o


-3o0 _
2, ,







2 3 4 5 6 7 8 910
Weeks
Mar. 8 1956 May 16 -
5




Fig. 7.-Average weekly minimum relative humidity and minimum tem-
perature, rainfall di tribution and disease development for the 1956 spring
season.

Although a heavy rain occurred during the first week follow.
ing seeding (before the seedlings emerged), less than a tenth
of an inch additional rain fell from that time until the latter part
of the fifth week. This was reflected in a low minimum relative
humidity. In addition, the weekly minimum temperature means






Northern Corn Leaf Blight in Sweet Corn


consistently ran below 60 F. (Fig. 7). These combined condi-
tions apparently were adverse enough to prevent primary infec-
tion. The rapid build-up of disease toward the close of the
season is attributed to timely rains, high relative humidity and,
especially, high minimum temperatures.
In 1957 the disease appeared as the seedlings were emerging.
It increased steadily throughout the season, resulting in a severe
epiphytotic. Reasons for this are obvious from Figure 8:


90


- 80
E

. 70
0


60


50


40
2


I 2 3 4 5 6 7 8 9 10
Weeks


-Mor. I I


1957


May 19 -


Fig. 8.-Average weekly minimum relative humidity and minimum tem-
perature, rainfall distribution and disease development for the 1957 spring
season.

From one to seven rains occurred each week, and the minimum
relative humidity did not drop below 62 percent after the first
two weeks. Likewise, the minimum weekly temperature did not
drop below 600 F. after the third week.
These observations suggest that minimum air temperature,
minimum relative humidity and rainfall markedly affect severity






Florida Agricultural Experiment Stations


of northern corn leaf blight in the Everglades. Perhaps the
most apparent of these is temperature. When weekly mean min-
imum temperatures were below 60 F., the disease was at a stand-
still. When the mean temperature was above 60 F. for pro-
tracted periods, the disease progressed. The rate of progression,
however, was definitely influenced by minimum relative humidity
and rainfall (Figs. 6-8).
As pointed out above, night-time relative humidity failed to
reach 100 percent on only two occasions. The fact that the dis-
ease failed to develop appreciably during extended periods (Figs.
6, 7), even though conditions of super saturation (dew/fog)
occurred practically every night, places importance on daytime
(minimum) relative humidities in respect to disease cycles. When
weekly minimum (daytime) relative humidities were 60 percent
or less, northern corn leaf blight failed to develop to any great
extent, although night-time relative humidities almost invari-
ably reached 100 percent. It would appear, then, that low day-
time relative humidities retard development of northern corn
blight in the Everglades, regardless of night relative humidities.
If this is true, the adverse effect of low relative humidity during
the day on sporulation and spore viability could be a plausible
explanation. Another less apparent one could be the effect of
low relative humidity on host tissues. It is conceivable that
growth of host tissues is retarded under such conditions, and
are thus less succulent. Infection, hence germ-tube penetration,
occurs much more frequently on succulent leaves in the whorl
than on the older unfurled leaves.
The number and distribution of rains appear to be as im-
portant as the amount in respect to disease development. The
late appearance of the disease in 1956 can be attributed largely
to dry weather from the second through the fifth weeks. Total
rainfall during that time was less than 0.1 inch (Fig. 7). About
3.4 inches fell during the comparable period of the previous year
(Fig. 6), but it all occurred during a 10-day interval in the third
and fourth weeks. Disease severity increased only slightly dur-
ing this period.
The correlation of northern corn leaf blight severity with
weather leads to speculation on the possibility of a disease fore-
casting program. Local variations in weather serve as one of
the chief obstacles in such a program, particularly in south Flor-
ida. A disease forecasting system should be possible, however,
if done on an individual basis by trained personnel. The savings






Northern Corn Leaf Blight in Sweet Corn


garnered through a flexible spray program based on such a pro-
gram could easily mean the difference between profit and loss
for the season.
A final noteworthy observation was made during this study.
It has been suggested that the stage of maturity of the corn at
the time infection occurs could have an important bearing on
yield (2). In 1955 when the disease was present throughout the
season, though never building up to severity rating in excess of
an index of 4.5, the yield reduction was estimated at 32 percent.2
Whereas, in 1956 initial appearance of the disease was in the
sixth week; final severity rating was 5.2-higher than the year
before. Still, yield reduction was only 22 percent." This agrees
with Eldridge's findings that leaf damage occurring mid-season
has a greater effect on yield than that sustained during the latter
part of the season (4). Only recently, Ullstrup (9) has ob-
served this same phenomenon on field corn. Under the highly
epiphytotic conditions of 1957, yield reduction in the Everglades
was estimated at 90 percent.2

SUMMARY

The severity of northern corn leaf blight during the spring
seasons of 1955, 1956 and 1957 was related to mean weekly min-
imum temperature, mean weekly minimum relative humidity and
frequency of rains. Disease was checked when the weekly mean
minimum temperature was less than 60' F., the mean weekly
minimum relative humidity less than 60 percent, and when rains
occurred less frequently than at weekly intervals.
Nozzle Type.-Three nozzle types tested performed equally
well under conditions of moderate disease. These were Myer's
Jumbo, Myer's Fembro and TeeJet T10.
Nozzle Placement.-Two nozzles directly over the row were
superior to one from the standpoint of both disease control and
yield. Beneficial effect of drop nozzles was not as striking as
that of the overhead placement, but an increase in disease con-
trol was obtained as the drop nozzles were increased from one
to three spaced on 15-inch centers.
Gallonage.-With a given concentration, more gallons per
acre for control are required under severe disease conditions than
under mild or moderate conditions. Where 150 gallons per acre
would be adequate under moderate conditions, 250 gallons per
The percentage figure was estimated from the reduction in yield in
untreated plots as compared to those sprayed with an effective fungicide.






Florida Agricultural Experiment Stations


acre might be necessary for most effective control under severe
conditions of disease.
Pressure.-With a constant gallonage output, disease control
was comparable with pressures of 200, 300 and 400 p.s.i.
Materials.-Based on performance and cost, the order of pref-
erence of the more effective materials is as follows: maneb, zineb
plus thiram, zineb, nabam plus zinc sulfate or Amobam and
Dyrene. Parzate (zineb) appeared to be more effective than
Dithane Z-78.
Phytotoxicity.-The neutral coppers caused a severe contact
burn and the antibiotics tested caused light to severe chlorosis,
stunt and lodging. Under conditions of prolonged free moisture
maneb, nabam (plus salts) and zineb-thiram mixtures caused a
contact burn when applied to young plants with leaves still in
the whorl.

LITERATURE CITED
1. CONOVER, R. Fungicidal control of Helminthosporium leaf blight of sweet
corn. Fla. Agr. Exp. Station Ann. Rpt., p. 328. 1953.
2. Cox, R. S. Control of the Helminthosporium blight diseases on sweet
corn in south Florida. Phytopathology 46 :112-115. 1956.
3. Cox, R. S., D. S. HARRISON and C. S. YAGER. A versatile spray rig for
small field plots. Plant Dis. Reptr. 39 : 48-50. 1955.
4. ELDREDGE, J. C. The effect of injury in imitation of hail damage on the
development of the corn plant. Iowa Agr. Exp. Sta. Res. Bul. 185. 1935.
5. HARRISON, D. S., and R. S. Cox. Spraying sweet corn for the control of
northern corn leaf blight. Fla. Hort. Soc. 68 :213-216. 1955.
6. Fla. Agri. Exp. Sta. Agri. Ec. Mimeo Rept. 57-6. pp 15-18. 1957.
7. HORSFALL, J. G., and R. W. BARRATT. An improved grading system for
measuring plant diseases. (abs.) Phytopathology 35 : 655. 1945.
8. STONER, W. N., R. A. CONOVER, J. M. WALTER, J. F. DARBY, G. SWANK,
JR., and F. V. STEVENSON. Control of two Helminthosporium leaf blights
of sweet corn in Peninsular Florida. Fla. Agr. Exp. Sta. Cir. S-69. 1954.
9. ULLSTRUP, ARNOLD J., and S. R. MILES. The effects of some leaf blights
of corn on grain yield. Phytopathology 47 :331-336. 1957.




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