VEGETABLE CROPS HOTLINE BULLETIN

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DOWNY MILDEW OF CUCURBITS - This disease has been observed in Hancock County on a susceptible variety of slicing cucumber. The diseased plants were observed in a downy mildew sentinel plot that is part of national effort to track and understand downy mildew. This is the first report of downy mildew in Indiana this year.

The outbreak reported here is of more importance to cucurbit growers who have crops that will be grown into the fall than those growers who will finish harvest in the next few weeks. All cucurbit growers in Indiana should strongly consider the management options described below. And please continue to let me know of any additional outbreaks.

Symptoms - Downy mildew is primarily a leaf disease. Often, the first symptoms one observes are yellow, angular or square looking spots on leaves. The underside of the leaves may be covered with a black fuzzy looking growth--this is the fungus that causes the disease. Leaves may eventually turn brown and crinkle. The leaves may turn upwards as they dry. Severe outbreaks may result in the rapid death of vines.

Disease cycle - The fungus that causes downy mildew has not been reported to over winter in Indiana; it “blows” in from southern states (or from greenhouses in Canada). Thus, we do not usually observe downy mildew until August or September.

Downy mildew requires a period of leaf wetness and high humidity for successful infection. Heavy dews can provide adequate moisture to get this disease going. Although the fungal spores may land in your field, there has to be leaf wetness for the disease to cause problems. The optimum temperature for downy mildew is 59 to 68?F. The cool weather and heavy dews much of Indiana has experienced recently are perfect for downy mildew.

Control - Consult the Midwest Vegetable Production Guide for Commercial Growers <www.btny.purdue.edu/Pubs/ID/ID-56/> for control measures. Briefly, contact fungicides such as chlorothalonil (e.g., Bravo®, Echo®, Equus®) or mancozeb (e.g., Dithane®, Penncozeb®) can be used against downy mildews. Recent data indicates that Curzate®, Previcur Flex®, Ranman®, and Tanos® have been effective systemic fungicides against downy mildew. Systemic fungicides will be more effective than contact fungicides. Please consult the label for important application and resistance management instruction. The weather-based disease-forecasting program MELCAST was not designed for downy mildew. Therefore, if downy mildew threatens, apply fungicides on a regular 5 to 7-day schedule.

For more information please contact:

Dan Egel, Extension Plant Pathologist
Southwest Purdue Agricultural Center
4369 N. Purdue Road
Vincennes, IN 47591
Phone: (812) 886-0198
Fax: (812) 886-6693
Email: