downy mildew, disease of plants, especially in cool humid regions, caused by several funguslike organisms of the phylum Oomycota. White, gray, bluish, or violet downy patches of mildew form mostly on the undersides of leaves in damp weather. Pale green to yellow or brown areas usually develop on the upper leaf surface opposite the downy growth. Affected leaves often wilt, wither, and die early. Stems, flowers, and fruits are sometimes infected. Seedlings may wilt and collapse. Garden plants, bush fruits, vegetables, and certain trees, shrubs, field crops, and weeds are susceptible. Downy mildew is commonly caused by members of the oomycete genus Sclerospora, but other pathogens include species of Bremia, Peronospora, Phytophthora, Plasmopara, and Pseudoperonospora.

Downy mildew can be avoided by rotating annual flowers and vegetables and by avoiding overwatering, overcrowding, and poorly drained soil. Other avoidance measures are growing resistant varieties, sowing disease-free seed, removing diseased parts and crop refuse, eliminating weeds, and maintaining balanced soil fertility. The application of copper or either of the fungicides maneb or zineb is effective against many downy mildews, but the amount of residue on vegetables must be considered.