Early symptoms appear as irregular, light green or yellow angular patches on upper leaf surfaces. Lesions may also be purplish red to dark brown. Under high humidity, downy mildews form fluffy or fuzzy (downy) areas of spores on the undersides of leaves. Under magnification, the downy areas look like very small bunches of branched hairs. Older leaves toward the bottom of plants are usually infected first and wither and brown as the disease proceeds up the stem.
Downy mildews are favored by low temperatures, and high relative humidity (90%+). Free moisture is required for germination and infection. Spores disperse via the wind. To control downy mildew, provide good air circulation and maintain low humidity. Avoid using overhead irrigation and water plant bases. Rogue affected plants. Purchase downy mildew-resistant varieties of plant stock. At the end of the growing season, remove and destroy infected plant debris which can serve as a source of spores for the next growing season.
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