Cucurbit downy mildew is a water mold or oomycete disease of cucurbit crops caused by Pseudoperonospora cubensis. Symptoms first appear on the upper leaf surface as angular, vein-bounded, yellow to pale-green spots, turning brown and coalescing to turn entire leaves brown with disease progression. In very humid conditions, the underside of leaves may appear fuzzy as the pathogen produces numerous spores which enable the pathogen to spread. The disease survives on living cucurbit plant tissue, and spores spread via wind, rain and irrigation splash, or human spread on equipment and hands.
Favorable conditions include high humidity and leaf wetness, as well as moderate temperatures. Cultural management involves planting resistant cultivars, avoiding overhead irrigation, and practicing wide row spacing to improve airflow. Knowledge of the cucurbit plant types affected can be very useful as there are now two known strain types, one which infects exclusively cucumbers and cantaloupes, and one which infects pumpkins, winter squash, and watermelons. The cucumber and cantaloupe strain type tends to have more fungicide resistance and aggressiveness.