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Early occurrence of downy mildew found on S.C. watermelon

Clemson University plant pathologist Anthony Keinath urges South Carolina farmers to scout and spray after downy mildew infection was found June 18 on non-sprayed sentinel watermelon at Clemson’s Coastal Research and Education Center (REC) in Charleston.

It is the earliest in the growing season that the watermelon strain of the disease has been found in South Carolina. The mildew also was found on cucumber, cantaloupe and several different varieties of squash over the past two weeks.

“The rainy cloudy weather we’ve been having favors rapid outbreaks of downy mildew. Early detection is critical to controlling outbreaks,” said Keinath, professor of plant pathology in Clemson’s School of Agricultural, Forest, and Environmental Sciences (SAFES).

Keinath advises all South Carolina growers of cucurbits, a family of plants that includes various squashes, melons and gourds, to immediately apply preventative fungicides and walk their fields twice per week to scout for the mildew.

Downy mildew leaf spots are more angular than round and usually have yellow halos. Keinath reports that this year’s leaf spots are smaller than what is normally seen, only an eighth to a quarter inch in diameter, and light brown. The mildew can be detected with the aid of a microscope or large hand lens as purple spores on the bottoms of leaves.

Instructions for mildew management can be found in Clemson Cooperative Extension Information Leaflet 90: Cucurbit Downy Mildew Management, or by contacting an Extension agent.

The sentinel plots at Clemson’s Coastal REC are part of the National Cucurbit Downy Mildew Forecasting project that is composed of 25 collaborating institutions in the eastern United States, California and Ontario, Canada, that report weather and other factors to a central online database.

Keinath is working with researchers at N.C. State to develop a model for predicting downy mildew infection risk to cucurbits.

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