Why do cucumber, cantaloupe, and watermelon leaves develop yellow spots and blotches on them?
Downy mildew is a disease of cucurbits that causes numerous small, yellow spots on the upper leaf surface. Over time, the spots will darken and infected leaves will die. Beneath these yellow spots, the fungus Pseudoperonospora cubensis, cause of downy mildew, can often be found as whitish, fuzzy growth on the leaf undersurface.
Downy mildew is favored by cool-warm, moist, overcast weather. Use varieties that are resistant to this disease. Fungicides are available for the home vegetable garden if disease becomes severe enough to warrant chemical control. Contact your local Extension office for fungicide recommendations.