Downy mildew was reported on a commercial cucumber crop in Allen County, Kentucky on July 23 and verified at the Plant Disease Diagnostic Lab in Princeton. Cucurbit downy mildew more often develops as a late season disease, but it has appeared in Kentucky in July for the second consecutive year. According to the Cucurbit Downy Mildew Forecast system, risk of the disease in Kentucky remains low, and thus far, no downy mildew activity has been detected in sentinel plots in Fayette and Breathitt counties. However, growers should monitor crops carefully since downy mildew can develop rapidly under favorable weather conditions.
Small, yellow, angular spots first become visible on the upper leaf surfaces (Figure 1) with slight water-soaking on the lower surface.
The pathogen sporulates on the lower leaf surface only, producing a light gray to white “fuzzy” (or “downy”) growth (Figure 2).
Early symptoms of downy mildew on cucurbits can be confused with other diseases, so it is important to have the disease confirmed if there is any doubt. Samples may be submitted for confirmation to the Plant Disease Diagnostic Laboratory in either Lexington or Princeton, KY through local county Extension offices.
Commercial growers who have not already started a preventive fungicide spray program should begin one as soon as possible and include general protectants for foliar fungal diseases as well as downy mildew-specific materials. Since downy mildew disease is caused by a “water mold” pathogen rather than a true fungus, the chemistries to control it differ from those used for other foliar diseases. Effective downy mildew products include Previcur Flex, Ranman, and Zampro. Pay close attention to label restrictions and PHI. Visit the Kentucky Pest News archives for a more detailed description of the disease, management practices, and fungicide information in the July 23, 2013 issue. Management information is also available in the UK Vegetable Production Guide for Commercial Growers (ID-36) available online and at county Extension offices.
By Julie Beale, Plant Disease Diagnostician and Shubin Saha, Extension Horticulturist