AGRICULTURAL RESEARCH CENTER
D 0 Hastings, Florida
SMimeo Report POL-71-10 September 1971
RESULTS OF 1970-71 TEST FOR CONTROL OF DOWNY
MILDEW IN CABBAGE SEEDBEDS AT HASTINGS, FLORIDA
D. P. Weingartner, Asst. Plant Pathologist
Downy mildew caused by Peronospora parasitica continues to be the most
serious disease of seedling cabbage in the Hastings area. The disease
severely retards growth of seedlings and in extreme cases kills them.
All commercially available cabbage varieties are susceptible to the
disease. Chloranil (Spergon) was for many years the best available
fungicide for controlling downy mildew. In recent years, however,
the general availability of chloranil has been questionable. Various
formulations of maneb have substituted for chloranil, but have not
always provided satisfactory disease control at the recommended rates.
In 1969, experiments were initiated with various formulations of maneb,
maneb-related fungicides and several new fungicides which had not been
extensively tested in the Hastings area. Rates of the organic fungicides
were doubled over those previously recommended for use. Results of the
1969-70 test were reported in POL Mimeo Report 71-1 Revised. Generally,
all of the organic fungicides tested gave excellent control of downy
mildew at the higher rates.
In 1970-71 fungicides which looked promising the previous year were
applied at three different rates and were compared to the recommended
rate of chloranil (4.0 lb. 65W/Acre).
Red Acre Cabbage which is highly susceptible to strains of Peronospora
occurring in the Hastings area was planted in seedbeds December 17, 1970.
Fungicides were applied in 100 gallons water/acre with a PTO driven sprayer
December 30, January 7-8, and January 11-12. Spray was delivered at 300 psi
via 6 nozzles/row. Eight drizzling rainfalls and cool temperatures occurred
during the test period. Conditions were ideal for development of downy
mildew which became epiphytotic. A hard freeze on January 21 killed all
those plants severely affected with downy mildew. Relative control of
downy mildew by the materials tested could therefore be readily assessed
by rating surviving stands of plants.
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The results are summarized in Table 1. None of the materials used,
including chloranil, satisfactorily controlled downy mildew. The
frequent rains apparently washed fungicides from the leaves leaving
the seedlings susceptible to attack by the fungus. High rates of
fungicides were generally more effective than low rates. Difolatan 4F
and Thynon gave the best disease control, however, neither compound
is currently registered for use on cabbage.
Data accumulated during the 1969-70 and 1970-71 growing seasons indicate
that adherence of fungicides to cabbage leaves rather than inherent
fungicidal or fungistatic activity limits effectiveness of these
chemicals in controlling downy mildew. Subsequent research will
emphasize evaluation of spreader stickers and other adjuvants in
increasing the life of fungicides on sprayed seedlings.
Maneb formulations (e.g. Manzate D 80W and Dithane M-22 Special 80W)
and Bravo 75W (each at 1.5 lbs/acre in 80-100 gallons water) are the
best available fungicides for mildew control in the Hastings area.
Even though up to 3.0 lb 80W maneb/acre is registered for use on cab-
bage, growers should apply this rate on a trial basis only until addi-
tional data on disease control, phytotoxicity and economics are avail-
able. Chloranil, pending its availability, is still registered for
use on cabbage. Chloranil should be used at 4.0 lb 65W/acre in 80-
100 gallons of water.
POL-71-10 Page 2
Table 1. Results of 1971 tests for fungicidal control of downy mildew.
Low Rate Stand Ratingl/ 2/
Treatment (LR) 1 x LR 2 x LR 3 x LR
Difolatan 4F 1.0 qt. 5.1 ab 5.2 ab 7.9 a
Thynon 75W + DuPont Spreader Sticker 1.0 lb. 3.0 b 3.0 b 5.3 ab
Polyram 80W 1.0 lb. 2.8 b 3.0 b 4.0 ab
Bravo 75W (Daconil 2787) 1.0 lb. 2.8 b 1.3 b 4.5 ab
Manzate D 80W + DuPont Spreader Sticker 1.0 lb. 2.5 b 2.1 b 4.5 ab
Dithane M45 80W + Triton B 1956 1.0 lb. 1.7 b 3.6 ab 3.1 b
TC 904 1.0 qt. 1.6 b 3.4 b 4.1 ab
Chloranil 65W + DuPont Spreader Sticker 4.0 lb. 2.9 b -- --
Check 1 -- 1.9 b -- --
Check 2 -- 2.0 b --
1/ Hard freeze on January 21 killed all plants which were stunted or severely
affected by downy mildew. Mildew control was therefore assessed by rating
plots on a 0-10 scale with 0 being all plants dead and 10 perfect mildew
control (perfect stand).
2/ Data are means of 4 replicates. Ratings followed by the same letter do
not differ significantly at 5% according to Duncan's multiple range test.
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