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Downy Mildew of Cucumber

From Illinois Fruit and Vegetable News: Downy Mildew of Cucumber

On September 7, 2013, downy mildew was detected in a cucumber leaf sample sent from Arcola (Douglas County, east-central Illinois). This is the first report of downy mildew on cucurbits in Illinois in 2013. On September 8, I checked several cucumber, melon, pumpkin, watermelon, yellow squash, and zucchini patches in Champaign Count...y, and no downy mildew was observed. The pathogen is likely to spread onto melon plants and may infect pumpkin and squash plants too. Downy mildew is not expected to impact yields in pumpkin, squash, and watermelon fields, as the season for these crops is coming to an end. Approximately 30% of processing pumpkin crops have been harvested, and harvest of jack-o-lantern pumpkins and winter squashes has already begun. Late-season cucumber fields should be sprayed at weekly intervals to prevent destruction of plants.

Downy mildew of cucurbits, caused by the oomycete Pseudoperonospora cubensis, is a very destructive disease of cucurbits. The pathogen that causes downy mildew of cucurbits does not overwinter in Illinois; instead it is blown into Illinois or brought in on plant material such as transplants. Downy mildew is favored by cool temperatures (59 to 68 F) and leaf wetness.

Downy mildew affects leaves only. The first symptom is usually the appearance of indistinct, pale green areas on the upper leaf surface. The pale green areas soon become yellow in color and angular to irregular in shape, bounded by the leaf veins. As the disease progress the lesions may remain yellow or become brown and necrotic. During moist weather the corresponding lower leaf surface is covered with a downy, pale gray to purple mildew. Often an upward leaf curling will occur. Several fungicides have been labeled for control of downy mildew of cucurbits, including products with the active ingredient phosphorus acid (Agri-Fos, Phostrol, ProPhyt), Curzate, Forum, Gavel, Presidio, Previcur Flex, Ranman, Revus, Tanos, and Zampro. In experimental plots in Illinois, spray application of Revus + Bravo Weather Stik (BWS), alternated with Ranman + BWS, Tanos + BWS, Gavel + BWS, or Zampro + BWS have been effective for control of downy mildew.

Mohammad Babadoost ( 217-333-1523

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