There are numerous fungi that fall under the general description of a powdery mildew. They will be treated together here since most of these have similar habits and similar management practices. 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 most frequently. 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 cause serious injury to the plants. Lilacs are able to survive year after year in spite of the disease. On the other hand, begonia may be seriously injured by even a mild infection. The tissue under the fungal growth dies soon after infection resulting 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. If powdery mildew has been a problem in the past, choose a variety that has some resistance to the disease.
Powdery mildew appears as a dusty white to gray coating over leaf surfaces or other plant parts (Fig. 1). In most cases this fungal growth can be partially removed by rubbing the leaves. It might be identified incorrectly as dust that has accumulated on the leaves. Powdery mildew, however, will begin as discrete, usually circular, powdery white spots. As these spots expand they will coalesce, producing a continuous matt of mildew (similar to dirt or dust). A plant pathologist using a microscope can determine whether a fungus is present anytime the whitish patches are present.
Symptoms usually appear late in the growing season on outdoor crops. The fungus is favored by periods of high relative humidity or site conditions that promote a more humid 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 includes stunting and distortion of leaves, buds, growing tips, and fruit. The fungus may cause death of invaded tissue (begonia, for example). 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 the growth and vigor of the plant. The seriousness of the disease will depend on the extent of the various types of injury.
The fungi which cause powdery mildew are spread by spores produced in the white patches. These spores are blown in the wind to other parts of the plant or to other plants 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 survives on infected plant parts and in debris such as fallen leaves. It may produce resting structures known as cleistothecia, which resist harsh winter conditions. These will appear as small black dots within the white powdery patches (Fig. 2). The next spring, sexual spores (ascospores) are released from the cleistothecia, shot up into the air, and carried by air currents to leaves of plants where new infections will begin. During the growing season, the fungus produces asexual spores (conidia) that help the fungus to spread and infection to build. This is the general cycle for most powdery mildews of outdoor plants. 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.