Clemson reports downy mildew detected in South Carolina; growers advised to spray cucurbits

CHARLESTON — Downy mildew has made its way to South Carolina and Clemson University Extension specialist Tony Keinath advises cucurbit growers to spray fungicides to cut their losses.

Downy mildew is a water mold that destroys plant foliage. It begins as dark, irregular spots that spread quickly on plants’ leaves, causing them to curl. A crop consultant found the disease June 6 on cucumbers in Bamberg County.

“Growers should spray all cucumber and cantaloupe crops to prevent or manage downy mildew,” Keinath said.

Downy mildew is common on cucurbits, a family of plants that includes various melons, squashes and gourds. Keinath encourages growers to spray with fungicides effective against downy mildew beginning now until harvest is over —as late as September in some areas of South Carolina. The three recommended fungicides are Ranman, Orondis Opti and Gavel.

The best fungicides are Ranman and Orondis Opti. Keinath advises using a two-way rotation with Ranman sprayed the first week and Orondis sprayed the second week. No downy mildew fungicide needs to be sprayed in the third week, he said.

“Reports from other states indicate that Orondis Opti is effective enough to last 14 days after application,” Keinath said. “Although Ranman is relatively inexpensive, it is being used very widely throughout the southern United States, which increases the risk of fungicide resistance to this product. I caution growers about relying too heavily on Ranman.”

Keinath said it is important growers remember these fungicides are effective only against cucurbit downy mildew. For more information, refer to Clemson Extension’s Cucurbit Downy Mildew Management for 2019 factsheet.

Downy mildew also has been reported on watermelons in south Georgia. Keinath advises South Carolina watermelon growers to spray their crops with protectants. For more information, refer to Clemson Extension’s Watermelon Spray Guide for 2019.

Keinath is a professor of plant pathology at Clemson’s Coastal Research and Education Center in Charleston.