Causal Agent: 

Oomycete (Pseudoperonospora cubensis)

Characteristic Symptoms:
 
Initial symptoms are semi-angular to irregular pale yellow spots on the upper leaf surface, which later turn brown or necrotic.
   
Under high relative humidity and cool conditions particularly in the early morning, sporulation appears as a fine, grayish to black molds on the undersides of the leaves.
   
Conditions for Disease Development:
 
The pathogen is an obligate parasite and infects only cucurbits.
   
The disease occurs in temperate and tropical areas with either high or low rainfall as long as sufficient leaf wetness periods are provided, usually by dew.
   
Sources of primary inoculum are airborne sporangia from either nearby or very distant infected cucurbits.
   
Sporangia are spread within the field by air currents, rain splashes or by contact with workers or tools.
   
High humidity (100% for 6h) with moderate temperatures (15-30°C) favours sporangia production.
   
Management and Control:
 
Use resistant varieties, if available.
   
Good field sanitation (i.e., remove and destroy infected leaves as early as symptoms are observed, remove weed hosts) can help reduce incidence and severity.
   
Provide good air circulation (avoid dense canopy) to reduce long periods of leaf wetness.
   
Avoid staggered planting.
   
Spray with chemical fungicides like mancozeb (e.g. Attain M-80Ⓡ, Achem Mancozeb 80 WPⓇ, Micron 80 WOⓇ, VanzebⓇ), metalaxyl+mancozeb (e.g. Ridomil Gold MZ 68 WGⓇ, Apron XL 350 ESⓇ), Pyraclostrobin (e.g. Cabrio 25 ECⓇ), or fosetyl aluminum (e.g. AlietteⓇ).
   
References:
Compendium of Cucurbit Diseases (1996) by the American Phytopathological Society
   

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