Powdery mildew is widely distributed and destructive. In cucurbits, the powdery mildew fungus first appears on old leaves as tiny, white superficial spots, followed by infection of stems and young leaves. As spots enlarge, they become covered with powdery spore masses. Under ideal conditions for disease development, including hot, dry weather, the entire leaf surface becomes covered with the spore masses and the leaves drop prematurely. Fruits are not infected but may have poor quality and texture because of the limited photosynthetic capacity of the reduced leaf area.
Powdery mildew is spread by wind and survives on weeds, and other crops, especially cucurbits.
Resistant cucumber and muskmelon cultivars are commercially available. Sulfur should be used on muskmelons very carefully because some cultivars will be damaged by this chemical. Subsequent disease development is very rapid, with the blisters coalescing and the entire infected area of the fruit becoming soft, watery, brown and covered with a fluffy white mat. Transition from spot to spoilage may occur within 48 hours under conditions of high humidity and standing water. In cucumber, cottony leak symptoms are often confused with Phytophthora fruit rot.