Worldwide distribution. On cucurbits, cucumber, melon, pumpkin, squash and zucchini. Some legumes and ornamentals are also susceptible to infection. An important disease.
White patches of fungus over both side of leaves. The fungus growers over theleaf surfaces, dropping structures ('haustoria') to feed on the leaves. Spores do not need water for germination, but humidity has to be high.
Leaves die early, fruits suffers sunburn, ripen early, do not store well, or have poor flavour.
Cultural control: tolerant varieties of cucumber, melon, pumpkin and squash; sites with good air circulation; avoid planting near old crops; weed; collect trash and destroy after harvest; crop rotation.
Chemical control: for organic production: horticultural oil, potassium bicarbonate, wettable sulphur; for non-organic production: wettable sulphur, chlorothalonil, trialzoles.