The fungus thrives in low temperatures and low humidity, usually between 64 and 75 degrees F. Temperatures higher than 86 degrees F will stop any powdery mildew from developing. Introduction can be from long range and sporadic, from either sucking insects or wind-blown spores.
A clear indicator of powdery mildew is the white patches of mycelium and conidia, which develop on the cotyledons, stems, pods, and upper leaf surfaces. Actual symptoms on the plant itself are interveinal necrosis (death), small specks on the leaf that are dead, and crinkling of the leaf followed by defoliation. At times, later in the season, plant leaves simply become yellow and drop off.