On Aug. 16, cucurbit downy mildew was confirmed on cucumber in south-central Kentucky. Cucurbit downy mildew has also been reported in neighboring states Tennessee and Ohio. Disease is likely widespread across the area.
Hosts & Symptoms
Cucurbit downy mildew is a serious foliar disease of cucumber, melon, pumpkin, and squash. Symptoms first appear as pale or bright yellow spots on top sides of leaves, particularly leaves near the crown of the plant. Over time, spots become irregular or "blocky" in appearance, delimited by leaf veins. Spots spread throughout the plant and quickly develop into necrotic (dead) lesions. During periods of high humidity, lesions on the underside of leaves may develop a gray to purple, downy like appearance.
Cause & Biology
Cucurbit downy mildew is caused by the water mold (fungus-like) pathogen, Pseudoperonospora cubensis. This pathogen is not known to survive in Kentucky during winter months. Living plant material is required for the pathogen to complete its life cycle. Thus, the pathogen overwinters in warmer locations in southeastern states. In summer months, spores are blown northward on wind currents or in storms.
Monitoring Disease in Kentucky
Incidence of cucurbit downy mildew is documented and monitored by the ipmPIPE Forecast Center. The Cucurbit Downy Mildew Forecasting page of the IPM pipe website provides information to growers regarding the spread and risk of cucurbit downy mildew. Cucurbit growers can sign up for free e-mail or text alerts to keep up to date on the proximity of downy mildew to their individual farm.
Management
Fungicides
Since cucurbit downy mildew has been confirmed in Kentucky, growers should include a protectant or systemic fungicide in their spray schedules. Mancozeb and chlorothalonil are good protectant products and are used as preventatives during summer months. Growers should regularly scout cucurbit fields and incorporate downy mildew-specific fungicides if downy mildew is confirmed in or near their location. These more specific products are more effective at managing downy mildew. Growers of high-value crops such as pumpkin, specialty gourds, and greenhouse cucumber should consider adding these products to their spray program now. The most effective products against cucurbit downy mildew include Orondis and Ranman; other options include Elumin, Previcur Flex, Tanos, Gavel, and Zampro.. More information on commercial fungicides effective against cucurbit downy mildew can be found in the University of Kentucky Vegetable Production Guide for Commercial Growers (ID-36) or the Southeast US Vegetable Crop Handbook (SEVEW). Home gardeners should continue to utilize chlorothalonil and mancozeb
Cultural Practices
Cultural practices that increase air circulation can help slow development and spread of disease. Plant location, spacing, and row orientation can help promote leaf drying, as moisture is the key driver of downy mildew. Weed management is also important for reducing leaf wetness. Avoid overhead irrigation, especially during evening hours. Sanitation practices should include destruction of infected plants as soon as possible to limit spread. Terminate cucurbit crops as soon as possible at the end of their production cycle; abandoned cucurbits can be sources of downy mildew for other vining vegetables within several hundred miles of the affected farm. Since the downy mildew pathogen requires a living host, an herbicide application is a fast way to terminate a cucurbit crop the producer would otherwise abandon at this point in the season. Other ways to terminate the crops quickly include physical removal with burial or burning.
Confirming Downy Mildew
If a grower suspects downy mildew in their cucurbit crop, they should contact their local county agent immediately to submit a sample to a Plant Disease Diagnostic Lab. Rapid confirmation of downy mildew in cucurbit crops is critical, not only to maximizing yield, but also protecting neighbors' crops.
Categories: Kentucky, Crops, Fruits and Vegetables, Ohio, Crops, Fruits and Vegetables