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CHEMICAL CONTROL OF POWDER NY DbEW OF SQUASH
IFAS, University of Florida, Everglades Reseach and Education
Center (EREC), Belle Glade.
Powdery and downy mildew continue to be two of the most
serious foliar diseases of summer squash grown in southern Florida.
These studies were carried out to evaluate several fungicides for
control of both powdery and downy mildew and to measure any yield
affects associated with disease control.
MATERIALS AND METHODS
Summer squash was mechanically seeded using a planter junior
into a Pahokee Muck soil (pH = 6.4) on the EREC farm on 24 February
1992. Summer squash cultivar Early Yellow Summer Crookneck was
used to plant four replications of each treatment in a randomized
complete block design. Test plots consisted of four rows, each 20
ft (6.1 m) long on 6 ft (1.8 m) centers. Fertilizer according to
soil test recommendations was broadcast and incorporated prior to
All test materials were applied with a CO2 powered backpack
sprayer at a pressure of 25 lb/in2 (17.24 newtons/cm ). The volume
of finished spray increased from 72 to 144 gal/acre (672-1344 L/ha)
as the plants increased in size. Larvin was applied weekly for
insect control using a self-propelled high pressure, high volume
commercial spray unit. Development of powdery mildew and downy
mildew was slow at first but by the last week of April there was a
substantial amount of both powdery and downy mildew. There was
also a much higher level of watermelon mosaic virus evident in the
beds than expected.
Experimental treatments, rates, and frequency of application
are shown in Table 1. Weekly sprays were applied 17 March, 24
March, 30 March, 7 April, 14 April, 21 April, and 29 April.
The severity of powdery and downy mildew was evaluated on 4
May. As coalescence made it difficult to discern individual foci,
estimates were made of the percentage of leaf surface covered with
mildew. Adaxial surface ratings are shown for powdery mildew and
downy mildew and abaxial ratings for powdery mildew only.
Plots were harvested twice a week from 9 April until 1 May.
All fruit from the two interior rows of each plot were picked and
graded. Records were taken of the weight of marketable fruit and
oversize fruit. Combined data from six individual harvests are
presented in this paper.
All disease severity and yield data were subjected to analysis
of variance, followed by a means separation according to Waller-
Duncan's methods. Percentage data were converted to arcsine square
root equivalents before analysis (1).
RESULTS AND DISCUSSION
Very little powdery or downy mildew was seen in plots before
first fruit set. The late onset of these diseases may explain the
lack of significant differences in yield between treatments.
The Bravo formulation seems to provide relatively good control
of both diseases. Disease ratings for downy mildew and powdery
mildew on the adaxial surface were lowest for Bravo 825 on 4 May
(Table 2). There were no significant differences between
treatments when powdery mildew was evaluated on abaxial leaf
surfaces. This may be related to the method of application of the
fungicides. With the nozzle arrangement on the boom of the
backpack sprayer, most of the fungicide spray was deposited on the
adaxial leaf surfaces.
Sulfur did not provide the control of powdery mildew seen in
some past tests (2,3). Further evaluations are needed to see if
sulfur is less efficacious for powdery mildew than previously
thought. Aliette provided fairly good control of downy mildew but
not of powdery mildew. The addition of a copper fungicide (Kocide
101) to the Aliette with appropriate buffering did not enhance its
activity against powdery mildew.
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.
3. Sonoda, R. M., and K. Pohronezny. 1983. Status and potential
of pesticide resistance among fungal and bacterial pathogens
of vegetable crops in south Florida. Proc. Fla. State Hort.
Table 1. Fungicide treatments evaluated for control of powdery
mildew and downy mildew of squash at Belle Glade, FL in
---Treatment ----- -Rate/100 gal
Treatment Rate/ba0 Qal
2) Kocide 101
3) Aliette + Kocide + K2CO0
4) Bravo 720
5) Bravo 825
6) Bravo 720 alt/w Omni
7) Super Six sulfur 6F
3 lb + 2 lb + 1.8 lb
1.5 pt alt/w 3.0 pt
8) Control (H20)
"All treatments applied with a CO2 powered backpack sprayer at a
pressure of 25 lb/in2, in a volume of water equal to 72-144
Disease ratings for powdery and downy mildew of squash
experiment, Belle Glade, FL 4 May 1992a
P= = =P= I =~= 1=-3 I- -= = =
4 Powdery Mildew
Adaxial Surface Abaxial Surface
% Downy Mildew
Super Six Sulfur
"Based 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
^ ~ ~ Ad x a su f c Ab x a qur--face-.- --- __ ,^,-.^ ^^_ ^ ^ ^^ __^__
Table 3. Yields in squash plots affected by powdery and downy
mildew at Belle Glade, FL 19920
aBased on twice weekly harvest from 9 April until 1 May.
The two interior rows (40 row ft) were used for yield
bIncludes oversized fruit.
cExpressed in kg/plot.