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S Ft. Pierce ARC Research report RL-1977-7.--- November, 1977
Effect of Several Fungicides on Onion and Onion Blas HU'Mi LIRAF
R. M. Sonoda and N. C, Hayslip APR26 1978
ABSTRACT IFA S. U
Onion blast (Botrytis squamosa) incidence was reduced by all fung- tdeas t
Dithane M-45, Difolatan 4F, Dyrene 65W and Bravo 6F. Bravo 6F (2 pts/100 gal)
treatments sprayed at 3-4 day and 7 day intervals had the fewest lesions. These
treatments, however, had the lowest yields and the smallest bulbs.
There is renewed interest in growing onion, Allium cepa, in south Florida.
Wehlburg et al. (1) list seven foliage diseases of option in Florida. Several
fungicides are registered for use as foliage sprays but little information is
available on their use or efficacy under Florida conditions.
The object of the work reported was to determine onion disease problems and
develop a program for their control.
MATERIALS AND METHODS
Six 4-ft-wide elevated beds, 7 ft between middles were prepared. Texas Early
Grano 502 and Granex 33 onion were seeded on Oct 22 '76 on alternate beds, eight
rows of each variety per bed, three beds per variety. The onions were sprayed at
5-6 day intervals with Dithane M-45 (1.5 lb/100 gal), Diazinon 4E (1 pt/100 gal)
and Nufilm 17 (1/2 pt/100 gal) from Nov 10 '76 to Jan 12 '77 with a tractor drawn
spray rig. Low temperatures on Jan 18-20 '77 caused some leaf damage. Stemphylium
spp. and Alternaria spp. were isolated from damaged tissue. Leaves produced after
the low temperature period were nearly lesion-free.
A split plot test with six treatments was set up after the low temperatures.
Treatments included four fungicides applied singly (Dithane M-45 80W, 2 lb/100 gal;
Difolatan 4F, 2 pt/100 gal; Dyrene 65W, 2 lb/100 gal; and Bravo 6F, 2 pt/100 gal);
a combination of Dithane M-45 80W, 1 lb and Dyrene 65W, 1 lb/100 gal; and untreated
control as main plots and two spray intervals (3-4 days and 7 days) as subplots.
There were two replicates of each treatment on Texas Early Grano 502 and two rep-
licates on Granex 33. NACO Spred-all, a spreader-sticker (6 oz. f1/100 gal) was
applied with each fungicide treatment. The subplots were 4 ft wide by 6.5 ft long.
A 5 ft wide swath was sprayed with a hand-held boom sprayer using Spraying Systems
D 4-23 disc type hollow cone nozzles with pressure maintained at 30 psi with a
regulator. Two plywood sheets (4 x 5 ft) were positioned at the end of the plots
to prevent spray overlap. Sprays were applied at 125 gal/gross acre. Diazinon 4E
was applied to all plots, at 7 day intervals with a tractor drawn spray rig.
Lesions incited by Botrytis squamosa were found on Feb 24 '77, Lesions on a
1 inch segment of the fourth from the youngest leaf, 5 inches above its point of
attachment to the bulb were counted on Mar 9 '77. All fungicide treatments reduced
incidence of leafspot on both Granex 33 (Table 1) and Texas Early Grano 502 (Table
Treatments sprayed with Bravo 6F had the fewest lesions. In most cases, fewer
lesions were found on plants sprayed every 3-4 days. However, a statistically
significant difference was found only for Dithane H-45 on Granex 33 (Table 1).
Bulbs of Granex 33 were lifted, topped and weighed in the field on March
22 '77. Significantly less yield was obtained from plots treated with Bravo 6F
as compared to other treatments (Table 1). Bravo 6F treatment also resulted in
the smallest bulbs. No statistical comparison was made for bulb size as plant
population per plot differed and plant population affects bulb size.
Texas Early Grano 502 was harvested and weighed on Mar 31 '77. Again, sig-
nificantly less yield was obtained for plots treated with Bravo 6F (Table 2).
Plots treated with Bravo 6F at the 3-4 day interval had significantly less yield
than those treated every 7 days, Bulbs from plots treated with Bravo 6F were
again the smallest.
The absence of yield enhancement with fungicide treatment as compared to un-
sprayed control can be attributed to the late appearance of j. squamosa in the
crop. The period between disease onset and almost total leaf destruction on un-
protected plants was short.
Although Bravo 6F, at the rates used, was apparently phytotoxic, further work
with the fungicide, at reduced rates, is warranted, since it was the most effective
compound against the disease.
1. Wehlburg, C. S., A. Alfieri, Jr., K. R. Langdon, J. W. Kimbrough. 1975. Index
of Plant Diseases in Florida. Fla. Dept. Ag. and Cons. Ser. Div. Plant Ind.
Bulletin 11. 285 pp.
Table I. Incidence of Botrytis squamosa and number of bulbs per plot, bulb weight and yield of
Granex 33 onion sprayed with fungicides at two spray intervals. /11-
2/ Spray Leafspots/inch 31 Wgt/bulb Yield (T/A
Fungicide and amt/100 aT interval subplot main plot # bulbs .(m) subplot ma
Bravo 6F 2.0 pt
Difolatan 4F 2.0 pt
Dithane M-45 80W 2.0 lb
Dyrene 65W 2.0 Ib
Dithane M-45 80W 1.0 Ib
; 4/- -
7/.3 a- 198
84.4 b l96
63.3 be .205
24.1 bc i:'199
I/ Planted on Oct 22 '76, spray trial started on Jan 26 '77, harvested:on Mar:227'77. -
2/ NACO Spred-all, a spreader sticker, applied with each Nungicidet:(6~oz.ifl./100 gal).
3/ Lesions counted on Mar 9 177. i : :
4/ Means followed by different letters are significantly different at 5% level (Duncan's multiple range
5/ Pairs of means followed by different letters are significantly different atbthe 5%7. evel.
Table 2. Incidence of Botrytis squamosa and number of bulbs per plot, bulb weight and yield of
Texas Early Grano 502 onion sprayed with fungicides at two spray intervals. I/
Fungicide and amt/lO0 gal-2
# bulbs (gm)
Bravo 6F 2.0 pt
Difolatan 4F 2,0 pt
Dithane M-45 80W 2.0 Ib
Dyrene 65W 2.0 Ib
Dithane M-45 80W 1.0 Ib
+ Dyrene 65W 1.0 Ib
7 days 18.7
3-4 days 16.8
1/ Planted on Oct 22 '76, spray trial started on Jan 26 '77, harvested on Mar 31 '77.
2/ NACO Spred-all, a spreader sticker, applied with each fungicide (6 oz. f1./100 gal).
3/ Lesions counted on Mar 9 '77.
1/ Means followed by the same letter are not different at the 5% level (Duncan's multiple range test).
5/ Pairs of means followed by different letters are --.:significantly different at the 5% level.