There are numerous diseases that fall under the general description of powdery mildew. Since most of these have similar habits and similar management practices apply, they will be treated together here. Houseplants that are commonly infected by powdery mildew include African violet and begonia. Outdoors there are numerous plants that may be infected in any year, but lilac, phlox, some rose varieties and fruit trees seem to be infected every year. The actual injury to the plant varies greatly with the species and even the variety attacked. For instance, lilacs are typically infected late in the growing season and this does not usually injure the plant seriously. Lilacs are able to survive year after year in spite of the disease. On the other hand, begonia is seriously injured by even a mild infection: The tissue under the fungal growth will die soon after infection. This may result in leaf drop and poor plant growth. Thorough management practices must be implemented in this case. Among roses there are varieties that are more seriously affected than others. You may choose a variety that will not be seriously injured.
SYMPTOMS: Powdery mildews appear as a dusty white to gray coating over leaf surfaces or other plant parts. Powdery mildew will begin as discrete circular, powdery white spots. As these spots expand they will run together producing a continuous mat of mildew which to the casual observer would appear similar to dirt or dust. Microscopic examination by a plant pathologist can distinguish the fungus at either stage when there is some doubt.
Normally, symptoms will appear late in the growing season on outdoor crops. The fungus is favored by high relative humidity periods or site conditions that promote this type of environment, such as close spacing of plants, densely growing plants, or shade. Indoors, symptoms may occur at any time of year, but the rate of spread and development will be affected by the relative humidity and temperature.
Injury due to powdery mildews may include stunting and distortion of leaves, buds, growing tips, and fruit. The fungus may cause death of invaded tissue as mentioned for begonia. Yellowing of leaves and death of tissue may result in premature leaf drop. Nutrients are removed from the plant by the fungus during infection and may result in a general decline in growth and vigor of the plant. The seriousness of the disease will depend on the extent of these various types of injury.
DISEASE CYCLE: The fungi which cause powdery mildew are spread around by spores produced in the white patches. These spores are blown in the wind to other parts of a plant or to other plants many times during the growing season. Generally each species of fungus will be limited in the number of plant species that can be attacked. For example, the species of fungus infecting lilacs will not cause powdery mildew on apples.
During the winter the fungus is able to survive on infected plant parts and in debris such as fallen leaves. The fungus may produce a resting structure known as a cleistothecium, which can resist harsh winter conditions. These will appear as small black dots within the white powdery patches. The next spring spores within the cleistothecia (ascospores) are shot up into the air and are carried by the wind to leaves of plants. This cycle is generally true for outdoor plants but with houseplants the overwintering stage is of little significance. Depending on the environmental conditions indoors, the fungus could continue to grow and spread during the entire year.
MANAGEMENT: For indoor plants disease management includes reducing relative humidity around plants and spraying with a fungicide. Pick a product with label directions for use on houseplants.
For outdoor plants, gather up fallen leaves in autumn and destroy them or put them out with the garbage. Where powdery mildew is a problem, resistant varieties should be grown when available. During the growing season, begin fungicide sprays when the first white patches are noticed. Repeat as indicated on the product label during cool humid weather. Management for fruit trees may be combined with the normal spray schedule and should begin at the green tip bud stage for apples.
Effective fungicides include Ferti-lome's Liquid Fungicide, Hi-Yield's Consan 20, and Safer's Garden Fungicide. Check the label for directions for the particular plant you are treating and cautions about sensitive plants. Do not use fungicides indoors if intended for outdoor use.
READ THE LABEL BEFORE APPLYING ANY PESTICIDE