Downy Mildew Found in Ohio Cucumbers: Growers Advised to Protect Crop

Editor: Photographs are available. Please contact Mauricio Espinoza, (330) 202-3550 or espinoza.15@osu.edu.

WOOSTER, Ohio — Downy mildew has been found on cucumber farms in northern Ohio, and Ohio State University vegetable experts are asking growers to take measures now to prevent damage from this potentially devastating disease.

Sally Miller, a plant pathologist with the Ohio Agricultural Research and Development Center (OARDC) and a vegetable-crops specialist with OSU Extension, said the fungal disease was confirmed in single fields in Medina County (June 21) and Erie County (June 25). Neither field had been treated with fungicides.

The Ohio outbreaks follow a report of the disease in southwestern Ontario, Canada, on June 8.

“We don’t know the source of the downy mildew spores in the Ohio outbreaks, but given the reports so far and the prevailing winds, growers with cucumber fields south and east of southwestern Ontario, as well as those near confirmed outbreaks in Ohio, should be particularly vigilant,” Miller said.

Miller is recommending that all cucumber growers in the northern part of Ohio step up scouting of their fields and protect their crop with fungicides. She said fungicides should be applied on a 7- to 10-day schedule under hot and dry weather conditions, and on a shorter schedule under cool, moist conditions.

Recommended products include Gavel (five-day pre-harvest interval, or PHI), Previcur Flex (two-day PHI), Tanos (three-day PHI) and Ranman (no PHI). These protectant fungicides must be tank-mixed with either Bravo (no PHI) or Dithane (five-day PHI). It’s also important to alternate these products to avoid development of fungicide resistance.

Organic growers have limited fungicide options available, but those in high-risk areas should apply OMRI-approved, copper-based fungicides to their cucumber crop on a weekly basis when weather conditions are favorable for the disease.

Last year, downy mildew cost cucumber farmers in Ohio and Michigan millions of dollars in fungicide application costs and crop losses.

In North America, downy mildew spores overwinter only in Mexico and the southernmost tier of the United States. The fungus can be carried northward by winds or storms, but Miller said a possible source of the current Ohio outbreaks may actually be greenhouse production in Canada.

“The weather in Ohio has been largely warm and dry this growing season,” Miller pointed out. “But Wayne and Medina counties have received approximately once-weekly thunderstorms, and the field in Huron County received about one inch of rain around June 4 and 1.4 inches on June 20. This may have created favorable conditions for movement of downy mildew spores and crop infection.”

Cucumber is the most susceptible of all cucurbit crops to downy mildew. But in the past, the disease has been spotted on squash and pumpkins several weeks after its appearance on cucumbers. Growers, Miller added, should also step up scouting of pumpkin and squash fields at this time and apply protectant fungicides if weather conditions are favorable, particularly if the disease has been reported in nearby cucumbers.

Caused by the fungal 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.

For up-to-date information about downy mildew, log on to http://www.ag.ohio-state.edu/~vegnet; contact Miller at (330) 263-3678, miller.769@osu.edu; or contact your county OSU Extension office.

OARDC and OSU Extension are the research and outreach arms, respectively, of Ohio State’s College of Food, Agricultural, and Environmental Sciences.

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