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Title: Chemical control of powerdy mildew of squash at Belle Glade, Florida in ...
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Creation Date: 1990
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6$
3~ EV-1990-4 June 1990

CHEMICAL CONTROL OF POWDERY MILDEW OF SQUASH
AT BELLE GLADE, FLORIDA IN 1990

KEN POHRONEZNY, MELANIE KEGLEY, MYRINE HEWITT, and DENISE
ROBINSON, IFAS, University of Florida, Everglades Research and
Education Center (EREC), Belle Glade

:ral Science
Library

iiT ODW4ION
Powdery mildew contintj obFdn4dne of the most serious
foliar diseases of iummer-squaue h-gwn in southern Florida.
These studies were carried out to evaluate several fungicides for
control of powdery mildew and to measure any yield affects
associated with disease control.


MATERIALS AND METHODS
Summer squash was hand-seeded into a Pahokee muck soil (pH =
6.1) on the EREC farm on 28 February 1990. Yellow straight-neck
cultivar Early Prolific was used to plant three replications of
each treatment in a randomized complete block design. Due to a
shortage of donated seed, the fourth replication was planted to
zucchini cultivar Elite 828. Test plots consisted of four rows,
each 20 ft (6.1 m) long on 6 ft (1.8 m) centers. The squash was
double cropped on plastic-mulched beds that had originally been
established 13 November 1989 for a pepper experiment. The pepper
plants were destroyed in the freeze of 25 December 1989, and the
stubble removed 3 January 1990. Prior to the pepper planting,
the soil was treated broadcast with 80 gal/acre (746L/ha) methyl
isothiocyanate (Vapam). An application of 1500 lb of 0-16-0
fertilizer amended with 40 lb Mn, 40 lb Zn, and 6 lb B was
broadcast and incorporated prior to bed construction.

All test materials were applied with a CO, powered
backpack sprayer at a pressure of 30 lb/in2 (20.69 newtons/cm2).
The volume of finished spray increased from 72 to 144 gal/acre
(672-1344 L/ha) as the plants increased in size. Maintenance
sprays of mancozeb and permethrin were applied weekly using a
self-propelled high pressure, high volume commercial spray unit.
Pressure from insects, downy mildew, and viruses was unexpectedly
light throughout the experiment. Natural development of powdery
mildew in plots was very slow. Therefore, leaves were collected
from a heavily diseased field in Homestead and spread throughout
the plots on 5 April. A substantial powdery mildew epidemic
ensued within 2 weeks.







Experimental treatments, rates, and frequency of application
are shown in Table 1. Biweekly sprays were applied 27 March, 10
April, 23 April, and 7 May. Additional dates for those
treatments sprayed weekly were 3 April, 17 April, and 30 April.

The severity of powdery mildew was evaluated weekly
beginning 18 April. Early ratings consisted of counting the
distinct mildew foci on the abaxial surface of five midcanopy
leaves taken from the middle rows of each plot. Later,
coalesence made it difficult to discern individual foci.
Estimates were then made of the percentage of abaxial or adaxial
surface covered with mildew. However, the levels of mildew were
very low on the adaxial surface on all dates. Therefore, only
abaxial surface ratings are shown.

Plots were harvested twice a week from 16 April until 9 May.
All fruit from the two interior rows of each plot were picked and
graded. Records were taken of the number and weight of
marketable fruit and oversize fruit. Combined data from seven
individual harvests are presented in this paper.

All disease severity and yield data were subjected to
analysis of variance, followed by a series of singles-degree-of-
freedom orthogonal contrasts. Data for number of foci were
converted to log and percentage data to arcsine square root
equivalents before analysis (1).


RESULTS AND DISCUSSION

Disease development was slow early in the course of this
experiment. Since cucurbits are not grown commercially in the
Glades, we suggest that inoculum levels of the powdery mildew
fungi are low in the area. Levels of disease increased
dramatically once the plots were exposed to the heavily diseased
leaves from Homestead. By the end of the experiment, disease
pressure was high enough to provide an excellent opportunity to
evaluate the efficacy of the fungicide treatments.

The sterol inhibitor fungicides, ASC-6681 and R10064B were
generally superior to the other fungicides (Tables 2-5). For
example, 886 mildew foci/5 leaves were recorded in control plots
on 26 April. Disease levels were reduced to less than 100 by
both rates of ASC-66811. The number in the 0.04 lb a.i. R10064B
treatment was 175. In contrast to last year's study (2), there
was no consistent superiority of the triadimefon/chlorothalonil
combination over the chlorothalonil alone (Table 3). The Bravo
720 formulation of chlorothalonil did not provide satisfactory
control of powdery mildew throughout the course of the test.

There was no statistically significant difference between
the two rates of ASC-66811 on any sampling date. Therefore, the
0.06 lb a.i. rate should provide adequate control under southern
Florida conditions at less expense than higher rates. On 26








April, the 0.04 lb a.i. rate of R10064B was statistically
superior to the 0.02 lb a.i. rate. Under moderate disease
pressure, the higher rate may be needed.

The highest yields recorded in this experiment were
associated with the best levels of powdery mildew control (Tables
4 & 5). About 100 more marketable fruit per plot were harvested
from the ASC-66811 treatments than from the controls. This
difference in number of fruit was equivalent to approximately 19
kg/plot or 7500 lb/acre.

The sterol inhibitor fungicides hold promise for the control
of this serious disease of summer squash in Florida. Registered
alternatives were not adequate in this field test. It is hoped
that registrations of these compounds will come about soon, and
that Florida growers will have them available for use.


LITERATURE CITED


1. Gomez, K. A., and Gomez, A. A. 1984. Statistical Procedures
for Agricultural Research. John Wiley & Sons. New York,
New York. 680 pp.

2. Pohronezny, K., Dankers, W., and Sherman, R. 1989. Chemical
control of powdery mildew of yellow squash at Homestead,
Florida in 1989.








Table 1. Fungicide treatments in powdery mildew squash
experiment, Belle Glade, FL 1990'


Treatment

1) Control (HzO)

2) ASC-66811

3) ASC-66811

4) ASC-66792 4C

5) ASC-66792 82

6) BRAVO 720

7) R10064B

8) R10064B


tnrn. n..~


)/2.2F

!/45DG


0.06 lb a.i.

0.12 lb a.i.

4.25 pts.

2.75 pts.

3 pts.

0.02 lb a.i.

0.04 lb a.i.


weekly

biweekly

biweekly

biweekly

biweekly

weekly

biweekly

biweekly


aAll treatments applied with a
a pressure of 30 lb/in2, in a
gal/acre.

bBiweekly is every other week.


CO powered backpack sprayer at
volume of water equal to 72-144


84tef1 Q gal Frequ y


T3*a/In oral









Table 2. Disease ratings for powdery mildew of squash experiment, Belle Glade, FL, 19908
----------------------------------------------


Sampling Date


18 April

Treatment No. Foci


26 April

No. Foci % Mildew


2 May

% Mildew


9 May

% Mildew


Control

ASC-66811 (Low)

ASC-66811 (High)

ASC-66792 40/2.2F

ASC-66792 82/4.5DG

BRAVO 720

R10064B (Low)

R10064B (High)


250

26

18

151

201

386

26

42


886

70

98

350

449

806

551

175


60.0

20.0

12.5

27.5

41.2

76.3

32.5

17.5


75.5

33.8

12.5

76.3

73.7

92.5

68.2

51.5


70.0

47.5

33.7

70.0

81.2

91.2

76.2

48.7


aBased on means of four replications of each treatment arranged in a randomized complete
block design. Ratings are composites for five midcanopy leaves taken from the interior
two rows of each plot.









Table 3. Preplanned single-degree-of-freedom orthogonal contrasts and F-test values for
number of powdery mildew foci on summer squash in experimental plots at Belle
Glade, FL, 1990a


F-Test Values

Sampling Date


Preplanned Contracts

Control vs. Treated

SI's'vs. Offer Fungicides

Chloroth. + Triad. vs. Chloroth.

Chloroth. D. vs. Chloroth. 720

ASC-66811 vs. R10064B

ASC-66811 Rates

R10064B Rates


18 April

No. Foci

5.16*

34.09**

1.22

0.65

7.4*

2.86

4.14


26 April


No. Foci

9.72**

25.04**

1.5

1.72

18.87**

0.08

6.8*


% Milderi


7.91*

14.80**

7.74*

6.34*

1.07

0.40

1.20


2 May

4 Mi 1 iw


2.69

26.78**

0.92

2.51

10.25**

1.84

0.97


*Contrast values for abaxial surface only shown. F-test from ANOVA not
significant for percentage of mildew on adaxial surface.

Denotes significant differences) at P < 0.05.

Denotes significant differences) at P < 0.01.


I- i ~I AdawL II








Table 4. Yields in squash plots affected by powdery mildew at
Belle Glade, FL, 1990"


YIELDb


Control

ASC-66811 (Low)

ASC-6681 (High)

ASC-66792 40/2.2F

ASC-66792 82/4.5DG

BRAVO 720

R10064B (Low)

R10064B (High)


Marketable

No. Weight0

172 29.63

260 40.38

271 42.06

226 32.95

203 33.15

199 28.54

210 32.73

210 33.02


Totalb

No. Weight

178 34.51

266 46.22

274 45.14

233 39.03

210 37.96

206 35.85

212 35.20

217 38.64


eBased on twice weekly harvest from 16 April until 9 May.
The two interior rows (40 row ft) were used for yield
determinations.

bIncludes oversized fruit.

CExpressed in kg/plot.


YIELD









Table 5. Preplanned single-degree-of-freedom orthogonal contrasts and F-test values for
marketable and total yield in powdery mildew of squash experiment at Belle
Glade, FL, 1990a


F-Test Values

Marketable Yield Total Yield
Preplanned Contrasts No. Weight No. Weight

Control vs. Treated 8.54** 5.08* 7.99* 2.87

SI's vs. Other Fungicides 4.58* 11.75** 3.80 2.81

Chloroth. + Triad. vs. Chloroth. 1.48 0.67 1.40 0.36

Chloroth. D. vs. Chloroth. 720 0.024 2.41 0.03 0.27

ASC-66811 vs. R10064B 10.66** 15.78** 9.92** 9.26**

ASC-66811 Rates 0.21 0.32 0.11 0.07

R10064B 0.0 0.08 0.04 0.72


"Based on twice-weekly harvest of interior 2 rows (40 row ft) of experimental plots.

Denotes significant differences) at P < 0.05.

Denotes significant differences) at P < 0.01.