Causal Agent:
Pseudoperonospora cubensis

Distribution:
Worldwide, especially tropical and subtropical areas

Symptoms:
The disease is most important on cucumber and cantaloupe, though all cucurbits are susceptible. Infected leaves first show a mottling followed by light yellow spots. These spots are angular and limited by the small veins of the leaf. Eventually these areas coalesce and turn tan to brown. A fine, white to gray, downy growth soon develops on the lower leaf surface. During periods of high moisture this growth may turn to a gray or purple color. Infected leaves die, but remain erect while the edges of the leaf blade curl inward. Severe infection results in defoliation, stunted plants and poor fruit development. 

Conditions for Disease Development:
The fungus is carried by air currents for considerable distances, splashed by rains or transmitted to healthy plants by workers and tools. The disease develops very rapidly in moderate to warm temperatures provided there is water on the leaves. Fog, dew or frequent rain provide conditions favorable for disease development.

Control:
Use resistant varieties when possible. For susceptible cucurbits a frequent spray program is important. A system called Blite-cast has been used successfully in some areas to predict disease occurrence and schedule spray applications.