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.