Preventing direct contact with the ground by using plastic or other mulches protects the fruit to some extent because this fungus is soil-borne. Cultivars differ in susceptibility, but none are resistant. Downy mildew, potentially a very serious problem in cucurbits, first appears as small yellowish areas on the leaves. When the leaves are wet overnight, spots may appear in the early morning as water-soaked, somewhat angular areas on the lower sides of leaves. The lesions change from yellow to brown as they enlarge, but retain an angular pattern of development.
Downy mildew can overwinter in some areas, but usually moves northward from subtropical areas.
Unlike powdery mildew, which grows rapidly in dry weather, downy mildew development is promoted by warm to hot, wet weather. The most practical method of control is the use of resistant cultivars. If fungicides are to be used, they must be applied with high pressure sprayers to assure complete coverage of foliage undersides. Good air drainage and growing in fields isolated from earlier plantings will also lessen disease severity. Rotation and reduced planting density to promote more open canopies should also be considered in areas with high incidence of this disease.