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Downy Mildew Now Confirmed on Many Different Types of Cucurbits in Pennsylvania

Posted: August 29, 2012

With the favorable cucurbit downy mildew weather over the past few weeks, downy mildew is being reported on many different types of cucurbits not just cucumbers and cantaloupes!
Characteristic downy mildew lesions on the upper leaf surface of pumpkin. On the lower leaf surface purplish-gray sporulation will develop under humid conditions. Sporulation may be more sparse in fungicide treated fields (Photo: Beth K. Gugino).

Characteristic downy mildew lesions on the upper leaf surface of pumpkin. On the lower leaf surface purplish-gray sporulation will develop under humid conditions. Sporulation may be more sparse in fungicide treated fields (Photo: Beth K. Gugino).

Cucurbit downy mildew has now been confirmed on all types of cucurbits grown in Pennsylvania including cucumbers, cantaloupe, acorn squash, butternut squash, giant pumpkin, pumpkin and watermelon with additional reports coming in daily.  In non-fungicide treated sentinel plots, downy mildew has also been observed to varying degrees on the two new downy mildew resistant cucumber cultivars just recently released from Seminis, SV3462CS and SV4719CS. However compared to the susceptible cv. ‘Straight Eight,’ symptoms were less severe and the disease progressed at a slower rate.  Conditions have been very favorable and much of Pennsylvania has been at high risk not only localized spread near infected fields but also longer distance transport across states. The regular application of fungicides is recommended for all cucurbit crops. If you have downy mildew in a field that you are no longer harvesting, it is recommended that you vine kill or disk in the field to reduce the potential spread of the pathogen to to neighboring farms.

Keep in mind that the purplish-gray sporulation characteristic of downy mildew may be more sparse and difficult to see on fungicide treated crops so it may be more difficult to determine whether or not your fungicide program is effective. In a year like this with such high disease pressure, scout your fields regularly and make notes as to whether or not the fungicides you are applying are effective. Although there are a number of reason why certain fungicides may be less effective (i.e. rate miscalculation, low sprayer pressure or spray volume, etc.), I would be interested in knowing what has (or has not) been working for you on your farm.

If you suspect downy mildew please continue to 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.