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Cucurbit Downy Mildew Update

Posted: August 27, 2014

Downy mildew has been confirmed on field grown cucumbers in Centre Co. and in greenhouse cucumbers in York, Co.

Downy mildew has now been confirmed on field grown cucumbers in Centre Co. and most recently in greenhouse cucumbers in York, Co. Although it has been suspected on other cucurbit hosts in several counties, those reports have all been negative so far. Since the update on Aug 21st, there have been new reports on cucumber in Ohio, cucumber, cantaloupe and watermelon in West Virginia, cucumber and cantaloupe in Maryland and cucumber in New York. Since last week, the forecasts have again become less favorable for disease development across PA.

Severe powdery mildew can sometimes mask the symptoms of downy mildew and it is not uncommon for both disease to be seen on the underside of the cucurbit leaves (see photo). Keep in mind that the fungicides that you apply to manage these diseases are very different and aside from the protectants such as chlorothalonil, mancozeb and copper, there are no products that will manage both diseases effectively.

Downy mildew can easily be confused with the bacterial diseases angular leaf spot or bacterial leaf spot, both of which have problematic in cucurbits this season. To distinguish downy mildew, look for the purplish-gray sporulation on the underside of the leaf. If you do not see downy mildew sporulation in the field (most obvious in the morning as the dew dries), place several symptomatic leaves in a sealed bag overnight and then check for purplish gray sporulation the next day.

If you suspect downy mildew on your farm to let me know either by email at bkgugino@psu.edu or by phone at 814-865-7328 or contact your local Penn State Cooperative Extension Office. For the latest information on outbreaks and to receive email or text alerts please visit the Cucurbit Downy Mildew Forecasting website (http://cdm.ipmpipe.org/index.php). Updates will also be made to the 1-800-PENN-IPM hotline weekly or more frequently if needed to provide growers with information that can be used to help make timely management decisions.

Contact Information

Beth K. Gugino
  • Associate Professor Vegetable Pathology
Phone: 814-865-7328