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Downy Mildew of Cucurbits—Pseudoperonospora cubensis

Downy mildew is caused by a fungus-like water mold which infects cucumbers, pumpkins, winter squash, and melons. It primarily infects the leaves, resulting in decreased photosynthesis leading to reduced yield and poor fruit quality; infections can be severe enough to kill the plant. The pathogen does not overwinter in the northeast, but is reintroduced each season, usually this occurs late enough to limit its effect.

downy mildew on cucumber leaf downy mildew on cucumber leaf downy mildew on canteloupe leaf
Downy mildew on cucumber–lesions are yellow and angular and confined by the leaf veins. The underside of the cucumber leaf shows fungal growth on the undersides of the lesions. Downy mildew on canteloupe–lesions are less angular than on cucumber and have a bright yellow halo.
downy mildew on watermelon leaf downy mildew on watermelon plant downy mildew on pumpkin leaf
Downy mildew on watermelon–lesions begin yellow and later turn brown or black. Leaves of watermelon plants will curl up on themselves. Downy mildew on pumpkin.

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[Photos, left to right: Gerald Holmes, Valent USA Corporation, Bugwood.org; Gerald Holmes, Valent USA Corporation, Bugwood.org; Gerald Holmes, Valent USA Corporation, Bugwood.org; Gerald Holmes, Valent USA Corporation, Bugwood.org; David B. Langston, University of Georgia, Bugwood.org; Paul Bachi, University of Kentucky Research & Education Center, Bugwood.org]

 
It is the policy of the State of Maine to minimize reliance on pesticides. The Maine Department of Agriculture and the Maine IPM Council encourage everyone to practice integrated pest management and to use pesticides only as a last resort. The mention of pesticides in the fact sheets linked to these pages does not imply an endorsement of any product. Be sure that any product used is currently registered and follow all label directions.