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\. 7 6L d'trobT of --Watermelon Diseases
D. L. Hopkins
The recommended fungicides (Table 1) will control the major foliar
fungus diseases of watermelon. The fungicide sprays may be applied
by various high-volume or low-volume ground sprayers and by airplane
sprayers. Regardless of the method of application, complete coverage
of the foliage is most important for good disease control. Inadequate
coverage results in poor disease control. The number and timing of
spray applications depend primarily on weather conditions. More
sprays are required generally in southern Florida than in central
and northern Florida. In southern Florida fungicide sprays are
necessary from seedling emergence; whereas, in northern Florida
regular fungicide sprays usually are not necessary until vining or
Gummy stem blight and downy mildew are currently the two most
prevalent and damaging foliar fungus diseases of watermelon in
Florida. The gummy stem blight fungus causes leafspots, stem
cankers, and fruit rot. All the recommended fungicides can provide
adequate control of gummy stem blight. In wet, rainy years when
this disease has been quite severe, Difolatan has been most
effective against it in tests at Leesburg (Table 2).
The downy mildew fungus attacks only the leaves of watermelon.
When environmental conditions are favorable, downy mildew develops
rapidly and may give an entire field a "burned-off" appearance.
Dithane M-45, Manzate 200, and Bravo have consistently provided
the best control of downy mildew in tests at the ARC, Leesburg
(Table 2). Benlate does not control downy mildew. The difficulty
in controlling downy mildew results from its rapid spread. It is
most important with this disease that fungicide applications are
made before the appearance of symptoms in the field.
With the widespread use of anthracnose-resistant commercial water-
melon varieties, anthracnose is not the serious overall problem
that it used to be. Where anthracnose race 2 does occur, it can
be controlled with the recommended fungicides. Alternaria and
Cercospora leafspots also occur on watermelon in Florida but are
not as severe as gummy stem blight and downy mildew. They are
more easily controlled with fungicides.
Fusarium wilt is caused by a soil-borne fungus and cannot be controlled
with fungicides. Infected plants wilt and usually die. The use of
resistant varieties along with planting on new land is the best
method of controlling, or preventing, this disease. If new land is
not available, a rotation of at least 6 years between watermelon
crops is recommended. Even new land can become infested with
Fusarium wilt through drainage water, cattle, or f Tpe
Leesburg ARC Research Report WG74-4 PR 1 197
January 23, 1974.
Bacterial leafspot is a disease which is associated with cool,
wet weather. This disease usually disappears with the onset
of warm dry weather, but if it becomes severe it can be controlled
with copper sprays (3 lbs. of 53% copper per acre).
Table 1. Recommended fungicides for the control of foliar
diseases of watermelon.
Fungicide Rate (amt./acre) to harvest1
Maneb 80% 1 1/2 Ibs. 5
Dithane M-45 80% 1 1/2 lbs. 5
Manzate 200 80% 1 1/2 lbs. 5
Difolatan ~ flowable 2 1/2 pts. NTL
Bravo 75% or 6F 1 1/2-2 1/2 lbs. or NTL
1 1/2-2 1/2 pts.
Benlate 50%2 1/4-1/2 lb. NTL
1 This is the minimum number of days allowed between the
last foliar application and harvest. NTL = no time
2 Benlate does not control downy mildew or Alternaria
Table 2. Fungicidal control of downy mildew (DM) and gummy
stem blight (GSB) of watermelon at Leesburg, Florida.
Rate % disease
(Amt./100 GS. DM 1971 yield
Fungicide gal. per acre) (1970) (1971) (tons/acre)
Dithane M-45 1 1/2 lbs. 39 18 26
Manzate 200 1 1/2 lbs. -- 27 24
Bravo 1 1/2 lbs. 45 35 25
Difolatan 2 1/2 pts. 22 53 26
Benlate + 1/2 lb. + 42 35 24
Manzate 200 1 lb.
Unsprayed -- 89 97 18
1 Ratings were made the third week of June in both years.