Downy mildew attacks all cucurbits, although cucumber is the most commonly infected. It appears first as small, pale green to yellow, angular spots delimited by leaf veins that give the foliage a mottled appearance. Eventually the spots coalesce and the leaf will turn brown. During moist weather, the lower surface of the leaf may be covered with a white to purple growth. Older leaves become infected first.
Identification
Downy mildew growth may initially be confused with powdery mildew. However, the two diseases differ in several important ways. Downy mildew produces spores mostly on the undersides of leaves and only after rain or very heavy fog. Spores disappear soon after leaves dry out. Powdery mildew does not require water on the leaf surface for spore formation, and the powdery growth appears on both sides of leaves. Generally, there is more mycelium found on plants infected with powdery mildew than on plants infected with downy mildew. Characteristic of downy mildew are the angular spots found on the leaves.
Solutions
Spores of the downy mildew fungus are carried by air currents or by rain-splash or sprinklers. A wet leaf surface is required for infection. Avoid using overhead sprinklers. Spacing plants to reduce canopy density and humidity will reduce spread. Some varieties of cucumber, melons, and watermelons may show resistance.
