Symptoms of powdery mildew include a white to gray mildew type of growth on the leaves, shoots, buds, flowers, or stems. This mildew is composed of thread-like mycelium and asexual spores of the fungus. The spores can be blown to other plant parts and cause further infection. The fungus is superficial, growing on the plant surface and sending structures into the epidermal cells. These structures obtain nutrients from the plant cells. New growth is particularly sensitive. The disease is very obvious and often unsightly. Occasionally infected foliage will exhibit a purple cast rather than a white color, as is true of infected apple or crabapple foliage or strawberry leaves.
Powdery mildew infection is favored by high humidity. The spores do not need a film of water to germinate and infect. Once infection has occurred, the mycelium on the leaf continues, rain or shine. Given additional high humidity, spores will continue to cause additional infection. The six common genera of powdery mildew fungi in the Midwest all prefer warm, humid days.
Powdery mildew is a common fungal disease problem on many perennials as well as annuals, shrubs, and even trees and turf. The most common hosts in Illinois seem to be lilac, zinnia, phlox, and rose, but certainly other species are affected. There are many different types of powdery mildew fungi, and most are very host specific. For that reason, we will probably never see an epidemic of this disease in Illinois. Still, on one plant the disease may spread very quickly, especially in humid weather. Although this disease does not kill plants, if your zinnias, roses, or other plants are infected, that may be a major aesthetic concern. Usually the powdery mildews in our landscape cause symptoms in mid-July, but high humidity in June this year has started the ball rolling early. The powdery mildew fungi on dogwood are active all summer.
To avoid problems with powdery mildew, provide conditions for adequate airflow in the planting. This may mean that plants need to be thinned or pruned to allow better air movement. Use recommended mature plant spacings when establishing new plants. Because the pathogen thrives in humid conditions, water plants early in the day to promote rapid drying. Avoid syringing foliage, and try to water the soil rather than the foliage.
Resistant varieties are the easiest means of disease control, but resistant plants are not always available or may not offer the flower color or size you prefer. Fungicides are available to control the mildews, and if sprays are begun at the first sign of mildew, control can be attained. Scout for the appearance of the disease and then treat the plants according to label directions. Often damage is minor and sprays are unnecessary. Consult the latest Commercial Landscape and Turfgrass Pest Management Handbook or the Home, Yard, and Garden Pest Guide for a list of registered fungicides by host and by disease. These manuals are available in your local Extension office and at http://www.PublicationsPlus.uiuc.edu. Report on Plant Diseases no. 617, Powdery Mildews of Ornamentals, is available at http://www.ag.uiuc.edu/~vista/horticul.htm or in Extension offices and provides detailed information about powdery mildew.