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College of Food, Agricultural, and Environmental Sciences


Ohio Sentinel Plots Track Downy Mildew

August 29, 2008

WOOSTER, Ohio -- With the success of Ohio State University's sentinel plots to monitor the development of field crop diseases, such as soybean rust and frogeye leaf spot, researchers are now using the system to track a potentially devastating vegetable crop disease.


Plant pathologists with the Ohio Agricultural Research and Development Center have established sentinel plots at four locations to monitor the development of downy mildew, a disease that attacks cucurbits (cantaloupe, cucumbers, honeydew melon, pumpkins, squash and watermelon). Cucumbers are especially susceptible to the disease.

Sally Miller, an OARDC plant pathologist, said that the sentinel plots are part of an Integrated Pest Management (IPM) downy mildew forecast system that is being implemented by North Carolina State University in collaboration with Ohio State University and other universities to help commercial producers track the disease and time fungicide applications based on disease development in and around their counties.

"The sentinel plots are an added line of defense to keep producers informed of downy mildew and how it may impact their crops," said Miller, a vegetable crops specialist with Ohio State University Extension. "Producers have to stay ahead of the disease. If it gets ahead of them, it probably wouldn't do much good from an economical standpoint to try and treat their crops."

Caused by the fungal-like organism Pseudoperonospora cubensis, downy mildew first appears as pale green areas on the upper leaf surfaces. These change to yellow angular spots. A fine white-to-grayish downy growth soon appears on the lower leaf surface. Infected leaves generally die but may remain erect while the edges of the leaf blades curl inward.

Miller said that the downy mildew forecast system Web site (North American Plant Disease Forecast Center: is helpful for both producers and home gardeners in identifying the disease, as its symptoms are similar to other diseases.

"Downy mildew looks different on cucumbers than it does on watermelons or cantaloupe, and its symptoms are similar to other diseases, such as angular leaf spot and anthracnose, as well as mite damage," said Miller. "Downy mildew has a way of fooling people."

The sentinel plots have been established at OARDC in Wooster, the North Central Agricultural Research Station in Fremont, the Muck Crops Agricultural Research Station in Celeryville and the Western Agricultural Research Station in South Charleston. Researchers plan to continue the sentinel plots in 2009.

According to the North American Plant Disease Forecast Center, nine Ohio counties have reported downy mildew outbreaks. Ohio producers are now harvesting their cucumbers, with 61 percent of the crop now harvested, according to the Ohio Agricultural Statistics Center. The first counties in Ohio to see the disease the last three summers have been the northern counties, particularly those bordering Lake Erie.

Downy mildew does not survive the winter in Ohio or other northern states, but it may be surviving in cucumbers produced in greenhouses in Canada.

"This 'green bridge' provides an early source of the pathogen – in late June or early July – in northern Ohio. Commercial growers and gardeners in that part of the state need to be the first to be on the lookout for downy mildew in cucumbers in early summer," said Miller. "Downy mildew was not found in the sentinel plot in South Charleston until mid-August."

Miller said that the first report of downy mildew on watermelon has just been announced and producers should now be keeping an eye on melons, squash and pumpkins. Fungicide application is the best way to control downy mildew, as researchers are still evaluating resistant varieties.

For more information on downy mildew developments, log on to the OSU Extension Vegetable Crop Web site at


Candace Pollock
Sally Miller