More reports of downy mildew on cucumber in Pennsylvania

Posted: July 18, 2012

In this past week, additional outbreaks of downy mildew on cucumber have been reported in Mifflin, Union and Lancaster Counties. It is also suspected in Snyder and Montgomery Counties.
Typical symptoms of downy mildew on the upper leaf surface of muskmelon. (Photo: Beth K. Gugino)

Typical symptoms of downy mildew on the upper leaf surface of muskmelon. (Photo: Beth K. Gugino)

In the past week, downy mildew outbreaks continue to be reported on cucumber in Mifflin, Union and Lancaster Counties in addition to the initial reports in Lebanon and York Counties. There is also an unconfirmed report from Snyder County. The weather patterns this past weekend put the eastern half of Pennsylvania at high risk for the spread of downy mildew. 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. Although our daytime temperatures have been high, cooler night temperatures, high humidity, and longer dew periods are favorable downy mildew.

On muskmelon, the foliar lesions are less angular than on cucumber. However on both crops, the lesions are initially are small and chlorotic and can appear water soaked early in the morning when dew is present. Downy mildew is confirmed by visually seeing purplish gray sporulation on the underside of the leaf.

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. At this time, the closest report of downy mildew on pumpkin is in a field in North Carolina.

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 ( 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.