Vegetable Disease Update – June 8, 2018

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland;

Scab of Cucurbits
Cool wet weather is associated with several vegetable diseases. One of these is scab on cucurbits, caused by the fungus Cladosporium cucumerinum, which is favored by temperatures at or below 70°F and wet weather.

Watermelons are highly resistant to scab, and many cucumber cultivars also have resistance (see the Commercial Vegetable Recommendation Guide for a list of resistant cucumber varieties). However, summer and winter squash, pumpkin, gourds, muskmelon and honeydew are susceptible. Symptoms vary somewhat depending on the cucurbit crop. However, leaf lesions appear as water-soaked, pale green to grey or white and angular. Dead leaf tissue cracks and looks “ragged” and is sometimes referred to as shot-holed. Fruit may have sunken spots (summer squash or susceptible cucumber), or raised scabby lesions (butternut squash). Scab should be managed by using resistant cultivars when available, and rotating out of susceptible crops (cucurbits) for two years. Application of chlorothalonil is also effective.

Scab lesions on summer squash

Scab lesions on cantaloupe

Hop Downy Mildew
Hop downy mildew was found near Wye Mills, MD on the Eastern Shore on Friday. Our weather remains very conducive for this disease which likes cool temperatures and high humidity. Symptoms are angular lesions on the leaves that are brown and necrotic. Image shows the upper surface of leaves with angular lesions caused by downy mildew. Sporulation occurs on the lower surface of the leaf.

Cucurbit Downy Mildew
Please note that the pathogen that causes hop downy mildew is NOT the same as the one that causes cucurbit downy mildew. Cucurbit crops should be monitored for presence of downy mildew, however the closest reported outbreak of cucurbit downy mildew as of June 7, 2018 is in North Carolina.

Image show a screen shot of current outbreaks of downy mildew in the eastern U.S. from the ipmPIPE website ( Red denotes counties where downy mildew was found in the last 7 days, and green counties represent outbreaks reported more than 7 days ago.

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