First symptoms are yellowish or light green areas on the upper surface of the leaves. As the lesions enlarge, the fungus produces downy growth on the underside of the leaf. Affected tissue turns brown, sometimes comprising extensive areas of the leaf. Infected leaves are usually invaded by secondary bacteria and fungi causing considerable damage during shipping. The fungus is rarely seed-borne and lives in the soil for very short times.
Infection takes place from diseased crop residue and is occasionally found on wild weed hosts.
Spores of the downy mildew fungus are wind-borne and are carried considerable distances. Spores germinate and infect leaves, producing symptoms within 5 to 10 days. Climatic conditions play an important role in the development of the disease. Cool, humid weather resulting from rain, heavy dew or fog is ideal for disease development. There are several races of the fungus and varieties differ in their reaction.