Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu
Watermelon downy mildew was confirmed in Wicomico County, MD on Aug 13, 2014. All watermelon crops in Maryland and Delaware, which will continue to be picked, should be protected with a downy mildew specific fungicide at this time. Downy mildew on watermelon looks different than downy mildew of other cucurbits (see Fig. 1.). This disease cannot overwinter here and must be reintroduced every year. This year has been very conducive to downy mildew, and our recent rain will increase disease.
Figure 1. Downy mildew on the upper (top) and lower (bottom) leaf surface. Lesions are dark brown, and irregular in shape. Larger lesions often have adjacent small, dark brown “dots.”
Downy mildew is managed best with fungicides that are targeted for downy mildew. Targeted sprays that are applied on a 7-day schedule will be most effective in managing the diseases. The following are effective fungicides when applied with a protectant fungicide such as chlorothalonil: Ranman, Previcur Flex and Zampro. Products that are very good for use in a tank mix as a rotation partner are Forum, Tanos, Gavel (Gavel contains mancozeb, which is a protectant, and does not need a tank-mix partner), Curzate, and Presidio. Remember to alternate among products in different fungicide groups.
Note: Downy mildew is now confirmed on pumpkin, cantaloupe, cucumber and watermelon on Delmarva. All cucurbit crops should be protected with downy mildew targeted fungicide applications.