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Update on cucurbit downy mildew

Posted: August 4, 2012

Downy mildew is widespread on cucumbers across Pennsylvania.
Typical symptoms of downy mildew on the upper and lower surface of pumpkin leaves. The observation of purplish-gray sporulation on the underside of the leaf is necessary for diagnostic confirmation. (Photo: Beth K. Gugino)

Typical symptoms of downy mildew on the upper and lower surface of pumpkin leaves. The observation of purplish-gray sporulation on the underside of the leaf is necessary for diagnostic confirmation. (Photo: Beth K. Gugino)

In the past week, downy mildew outbreaks continue to be reported on cucumber in PA. Within the last few days it has also been reported on cantaloupe on Long Island and additionally on giant pumpkin in north central Ohio. The scattered storms this past Saturday as well as today placed much of PA a moderate risk for disease spread. It is very important to be scouting all cucurbit crops regularly especially cucumbers and muskmelons. Symptoms from infection this past week could be visible in as few as 3 to 5 days. The unsettled weather will continue to put cucurbit crops at risk.

Tank-mixing Ranman or Previcur Flex with a protectant fungicide and alternating between protects with different modes of action (FRAC codes) is recommended for cucumber. It is important not to rely on one fungicide class. Since downy mildew does not overwinter in our region, it is always coming from somewhere and therefore it can be difficult predict which fungicides will be most effective during the growing season. Thinking proactively about managing for resistance will improve your ability to manage downy mildew on your cucurbit crops. For a more complete list of fungicides for managing downy mildew consult the Commercial Vegetable Production Recommendations for 2012. For a list of organic recommendation see Cornell Organic Cucumbers and Squash Production Guide.

If you suspect downy mildew please contact your local Penn State Extension office and/or send me a sample (overnight delivery) for confirmation to Beth Gugino, Department of Plant Pathology, 219 Buckhout Lab, University Park, PA 16802. We will examine the sample under a microscope and look for the characteristic downy mildew spores.

Please visit the Cucurbit Downy Mildew Forecasting website (http://cdm.ipmpipe.org/index.php) for the latest list of disease outbreak locations and forecasts. This information is updated by the end of the day on Monday, Wednesday and Fridays.