Powdery mildew first appears around midsummer in the fields in Illinois. The disease can develop in greenhouse at any time. All foliar tissues can be infected. Major symptom is small, circular, talcum-like spots on vines and leaves that gradually expand and may coalesce superficial, powdery, white to gray fungal growth develop that later covers part or all of the upper leaf surface, petioles, and stems. This growth also may appear on the underleaf surface and uncommonly on the fruit. Severely infected leaves gradually turn yellow, then wither, die, and finally become dry and brittle. Premature loss of the foliage results in yield losses and poor fruit quality.
In Illinois, the fungus is presumed to overwinter in crop and weed refuse. More important sources of infection may be spores produced on greenhouse-grown vine crops and perennial hosts grown in the field in frost-free areas of the far South during the winter. These spores are believed to be blown progressively northward during the spring and early summer. The powdery mildew fungus is a bit of an oddball as far as fungal diseases go because its development is limitedby wet leaf surfaces. However, high relative humidity and temperatures around 80 F favor disease development. The fungus can reproduce and infect with a relative humidity as low as 46 percent.