An uncommon and damaging strain of cucurbit downy mildew has been seen in Charleston County, S.C. The disease can damage crops such as cantaloupe, cucumber, summer and winter squash, pumpkin, watermelon and bottle gourd.

The disease is also widespread in the middle part of the state on cucumber, summer and butternut squash, according to Anthony Keinath, Clemson University vegetable pathologist at the Coastal Research and Education Center in Charleston.

Keinath said the symptoms on watermelon are different than symptoms on other cucurbits. Leaf spots on watermelon are dark brown and irregular in shape. Slight yellowing may be seen around the edges of the spots or in small patches in other parts of the leaf. Leaves infected curl inward as the leaves die.

Spores are usually found on the bottom of the leaf, although spores may be formed on top of the leaf in severe infections.

The best time to look for spores is in the morning before dew has dried. The spores are brownish-purple and the mold growth is white to colorless.

Keinath suggests looking at the size, shape and position of leaf spots to distinguish downy mildew on watermelon from gummy stem blight, which is the most common leaf disease on watermelon. Leaf spots of gummy stem blight are larger than individual spots of downy mildew.

Downy mildew kills leaves and vines prematurely, which reduces the number and size of the fruit but does not cause it to rot. It also reduces the sugar content of fruits, particularly cantaloupe and watermelon.

“All cucurbit crops in South Carolina are at serious risk,” Keinath said. “Growers should alternate Tanos (a fungicide from DuPont) with Previcur Flex (a fungicide made by Bayer). Both should be mixed with chlorothalonil or mancozeb.

Crops must be sprayed every five to seven days.”

Keinath suggests that organic growers use copper products but also should expect yield loss, because copper is not completely effective against downy mildew.

Home gardeners should spray with chlorothalonil (daconil) or copper products.

For more information, contact Keinath at (843) 402-5390 or tknth@clemson.edu or call your local Clemson Extension office.