Several different fungi
cause downy mildew disease on vegetables, fruits, ornamentals, forages, field crops and tobacco. These include Albugo
spp. (crucifers), Bremia
spp. (lettuce), Peronospora
spp. (cabbage, tobacco, spinach, soybeans, alfalfa, onion, many ornamentals), Plasmopara
spp. (grape and sunflower), Pseudoperonospora
(sorghum and maize), Sclerospora
(grasses, millet), and Sclerophthora
(maize, rice, wheat).
Downy mildew fungi are fairly host specific. The downy mildew fungus that infects one type of plant (e.g., rose) is not the same downy mildew fungus that infects another (e.g., grape). However, if you see downy mildew on one plant, then environmental conditions (i.e., cool, wet weather) are favourable for development of downy mildews on a wide range of plants.
Downy mildew of grape, spinach, and tobacco causes serious economic losses. It spreads rapidly through fields and is dependent on a wet, humid environment with cool or warm, but not hot, temperatures. A film of water is needed on plant tissue for spore germination and infection.
Conditions that favour development include: