Powdery Mildew

Pathogens: Sphaerotheca pannosa and Podosphaera tridactyla

Symptoms, Life-cycle and pest management

Powdery Mildew can attack plants at any stage of maturity, but it is most prevalent in late summer and early fall when nights are damp and cool and days are warm and dry. Long periods of relatively dry summer weather favor its development and spread; frequent rains discourage it.

Small patches of fine, white to pale gray, cobwebby growth develop on the upper leaf surface. The patches later enlarge and merge, and the leaf surface looks as if it has been dusted with white flour. The fungus may also grow on the lower surface of the leaves and on the stems. Severe attacks can make entire fields appear white. Infected leaves may turn yellow and wither prematurely.

The powdery mildew fungus survives the winter on diseased plants as fungus fruiting bodies (cleistothecia) that appear as black specks. Ascospores are produced within asci in the cleistothecia and provide primary inoculum. The ascospores are released when the cleistothecia and asci split open and are blown to leaves. Infections can occur at any time during the growing season, but are most common from midsummer to early fall.