Downy Mildew Making Early Appearance In South Carolina

From Clemson University
Downy mildew is making its annual appearance in South Carolina a little early this year and a Clemson University specialist says now is the time to protect cucurbits.
Downy mildew destroys plant foliage and is common on cucurbits, a family of plants that includes cucumbers, various melons, squashes and gourds. It makes annual unwelcomed visits to South Carolina in late May or early June. The first sighting for 2023 was on butternut squash in Charleston on May 12.
“This is two weeks earlier than when it was first detected in 2022,” said Tony Keinath, professor of plant pathology, and Research and Extension vegetable pathologist at Clemson’s Coastal Research and Education Center in Charleston, South Carolina. “The strain that affects squash and watermelon often is not seen until later in the year.”
Keinath credits downy mildew’s early appearance with South Carolina’s spring temperatures.
“The moderate spring temperatures we’ve had may be responsible since the downy mildew organism doesn’t like extremely hot weather,” he said. Growers should not be alarmed by this early appearance but should spray all cucurbits with one of the fungicides recommended in the Cucurbit Downy Mildew Management spray guide available from the Clemson Cooperative Extension Service’s Land-Grant Press.
“Growers also should be prepared to start applying downy mildew fungicides weekly after the disease appears in their area,” he said. To stay updated, growers can get cucurbit downy mildew forecasts at
Cucurbit downy mildew is caused by the fungus-like oomycete Pseudoperonospora cubensis. It does not overwinter in South Carolina but spreads from more southern states via wind-blown spores and can travel up to 1,000 miles in two days. This pathogen also can be spread through infected transplants. The disease begins as dark, irregular spots that spread quickly on plants’ leaves, causing them to curl. It ultimately leads to a reduction in photosynthesis that can interfere with fruit development.
In addition to South Carolina, Downy mildew also has been reported on watermelons in Florida and squash and cucumbers in southern Georgia. Keinath advises South Carolina watermelon growers to spray their crops with protectants. Spray recommendations can be found in the
Watermelon Fungicide Guide, also available from Clemson Extension’s Land-Grant Press.
For questions about cucurbits or other crops, contact the local Clemson Extension office.

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