The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source
site maintained by the Florida
Cooperative Extension Service.
Copyright 2005, Board of Trustees, University
S.i. Ft. Pierce ARC Research Report RL-1978-6
CONTROL OF ONION BLAST, RESULTS OF 197 '"I'/i( v
R. M. Sonoda and N. C. Hayslip/ .S..
Foliar sprays with Benlate 50W, Difolatan 4F, Dithane M-45 80W or Bravo 6F,
reduced incidence of onion blast incited by Botrytis Squamosa J. C. Walker on
'Granex 33' onions. Use of these fungicides reduced yield losses in 'Granex 33'.
Dithane M-45 80W, Bravo 6F, Difolatan 4F and Kocide 101, reduced onion blast
incidence and decreased yield losses due to the disease in the variety 'Texas
Early Grano 502'. Pre-harvest sprays had no effect on post-harvest development
of Aspergillus sp. or bacterial rots on bulbs.
Onion blast incited by Botrytis squamosa J. C. Walker has been one of the
most common disease problems of onions grown at the Agricultural Research Center,
Fort Pierce (ARC-FP). Lesions caused by the pathogen were observed in commercial
onion plantings in south Florida in 1977-78. The disease is found in other parts
of the United States (1). Applications of foliar sprays have been effective in
controlling this disease in other parts of the U. S. (1) and several fungicides
are registered for this use. Until recently (2), no data were available on the
effectiveness and suitability of new fungicides for use on Florida onions. Part
of the reason for this lack of information was that very few onions were grown
in south Florida. A recent renewal of interest in growing onions in south
Florida has resulted in a need to determine which diseases are important and
what measures are required to control them.
Tests at the ARC-FP in 1976-77 showed that disease incidence was reduced
by several fungicides (2). Results showed that Bravo 6F was most effective in
reducing disease incidence, but plots sprayed with this fungicide had smaller
bulbs and less yield than other plots. Since the pathogen moved into the crop
late in the season, there was no reduction in yield for the untreated plot.
The tests reported below were undertaken to determine 1) the effect of
Bravo 6F on yield; 2) if yield losses can be reduced with fungicide sprays;
and 3) if pre-harvest fungicide sprays have any effect on post-harvest disease.
Materials and Methods
Onions (Allium cepa L.), varieties 'Granex 33' and 'Texas Early Grano 502',
were seeded five rows to a bed on 42-inch-wide beds in Oldsmar fine sand on
Nov. 7, 1977 at the ARC-FP. The beds were fertilized before planting and
fertilizer applied as needed during the crop. The plots were sub-irrigated.
Foliar sprays with the fungicides listed in Table 1 and Table 2 were applied
at weekly intervals with a hand-held bloom-type sprayer delivering spray at
30 lb/in beginning Jan. 4, 1978. There were five replications of each treat-
ment in a randomized block design. Each plot was 10 ft. long with 5 feet space
between plots down row.
1/ Associate Plant Pathologist and Horticulturist, respectively, University of
Florida, Institute of Food and Agricultural Sciences, Agricultural Research
Center, Fort Pierce.
The number of lesions on a 1 inch segment of the third from youngest leaf,
6 inches above its point of attachment was counted on March 29, 1978 on 'Granex 33'
and April 3 on 'Texas Early Grano 502'. 'Granex 33' bulbs were harvested and
weighed on April 18, 'Texas Early Grano' bulbs on April 25. Thirty to 40 lbs of
bulbs were stored in crates in an open barn and counts of diseased bulbs made
Results and Discussion
Onion blast incited by B. squamosa was severe. A few lesions of Stemphylium
spp. were found scattered lightly on a few plants throughout the plots. Benlate
50W, Dithane M-45, Difolatan 4F and the three Bravo 6F treatments significantly
reduced incidence of B. squamosa on 'Granex 33' as measured by leaf spots/inch
(Table 1). Yields from these same treatments were significantly better than from
untreated plots. The treatment with Benlate 50W had significantly higher yields
than the Bravo 6F treatments. All Bravo 6F treatments had about the same yield.
Granex 33' bulbs were significantly larger in the better treatments than in the
Plots of 'Texas Early Grano 502' treated with fungicides had fewer lesions
and higher yields than the untreated plots (Table 2). The size of bulbs was
significantly larger except for the Bravo 6F 2 pt treatment. Kocide 101 performed
better on 'Texas Early Grano 502' than on 'Granex 33'.
Two compounds registered for use on onions, Dithane M-45 80W and Difolatan 4F,
were effective in reducing losses due to B. squamosa. Bravo 6F performed well and
did not reduce yields as opposed to the results from the tests of 1976-77. Bravo
6F is registered for use on all onions except the types described as 'sweet
spanish'. Further tests of this compound on 'Granex 33' and 'Texas Early Grano 502'
are needed. Benlate 50W is not registered for use on onions.
S There was no difference between any of the treatments in incidence of post
harvest storage problems on either variety of onion. The most prevalent pathogen
was Aspergillus sp. Less than 22% of 'Texas Early Grano 502' bulbs were rated
disease-free by July 11. Most of the affected bulbs had Aspergillus sp. on them.
1. Shoemaker, P. B. and J. W. Lorbeer. 1977. Timing initial fungicide application
to control Botrytis leaf blight epidemics on onions. Phytopathology 67:
2. Sonoda, R. M. and N. C. Hayslip. 1977. Effect of several fungicides on onion
and onion blast. Ft. Pierce ARC Research Report RL 1977-7. 4 pp.
Table 1. Effect of weekly foliar sprays with fungicides on B. squamosa
incidence, yield and bulb size of 'Granex 33' onions.
Funeicide and rate/100 salsfi/
Benlate 50W 1#
Dithane M-45 80W 2#
Difolatan 4F 2 pts
Bravo 6F 2 pts
Bravo 6F pt
Bravo 6F 1 pt
Kocide 101 I#V
Kocide 101 2#
Wettable sulfur 2#
1/ Naco Spred-all at 6 fl. oz/100 gal added to all fungicide sprays.
2/ Number of lesions per inch of leaf.
3/ Means in a column followed by the same letter are not different (DMRT P:.05).
Table 2. Effect of weekly foliar sprays with fungicide on B. squamosa
incidence, yield and bulb size of 'Texas Early Grano 502'.
Fungicide and rate/100 gals1/ DSI lb/bulb Tons/acre
Dithane M-45 80W 2# 10 a3 0.59 a 28.3 a3/
Difolatan 4F 2 pt 14 ab 0.59 a 27.0 ab
Bravo 6F 2 pt 9 a 0.39 b 26.0 abc
Bravo 6F 1 pt 16 ab 0.54 a 24.6 be
Kocide 101 1# 19 b 0.40 b 23.5 bc
Bravo 6F pt 22 b 0.44 b 23.4 bc
Kocide 101 2# 9 a 0.44 b 23.1 c
Control 32 c 0.37 b 19.7 d
1/ Naco Spred-all at 6 fl. oz/100 gal added to all fungicides.
2/ Number of lesions per inch of leaf.
3/ Means in a column followed by the same letter are not different
(DMRT P .05).