CLEMSON, S.C. — The water mold cucurbit downy mildew affects cucurbits (vine crops in the squash family) in South Carolina every year, especially in summer and fall. Downy mildew spreads quickly on cucumber, cantaloupe, and watermelon. Prevention before it appears, and prompt action afterward, are needed to manage this aggressive pathogen.

Symptoms and Signs

Leaf spots on cucumber or cantaloupe start as pale green to yellow, angular spots that turn brown. Leaf spots on squash and pumpkin are small, bright yellow flecks across the leaf surface that enlarge and turn brown. Symptoms on watermelon are quite variable. Sometimes spots are small and yellow, and in other cases, they are up to one-half inch wide, irregular, and brown. Brownish-purple spores are found in patches on the bottom of infected leaves in the early morning.

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–Clemson Extension Land-Grant Press