Powdery mildew is the common name for the disease and symptoms caused by a closely related group of fungi. These fungi grow on the upper and lower leaf surfaces, young stems, shoot tips, flower buds, and/or blossoms of plants. As they grow, they produce microscopic chains of spores that give infected areas their characteristic white powdery appearance.
The fungi parasitize the tissues of the plant causing a decline in its vigor. They also block light needed for photosynthesis. Infection is rarely lethal, but does cause leaf yellowing and browning, leaf distortion, premature leaf drop, and blemished or aborted flowers and slower-than-normal growth. Young plants grown in heavy shade are the most seriously affected by this disease.
The optimum conditions for powdery mildew development are warm days followed by cool, humid nights. Dry daytime weather allows spores to spread to other plants on air currents. On a cool evening, they absorb enough moisture from the air to germinate and cause infection. The entire powdery mildew life cycle can take place in less than a week under ideal conditions, and many overlapping infection cycles can occur within a single growing season. These fungi overwinter in the bud scales for initiation of infection next season.
Annuals/Perennials commonly troubled by powdery mildews include aster, centaurea, coreopsis, lathyrus, zinnia, monarda, phlox, and rudbeckia. Selection of resistant varieties will also help to reduce the occurrence of powdery mildew.
Control begins with selection of plants resistant to powdery mildew. Place susceptible plants where there is adequate sunlight and good air circulation to reduce humidity levels. Allow proper plant spacing for the same reasons. Pruning for better air circulation also may help. Check the label registration on horticultural oil products for powdery mildew control listings but do not use if temperatures are above 85 degrees F.