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Cornell University : Cornell Cooperative Extension
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| Yates Association |
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Powdery Mildew |
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There are numerous diseases that fall under the general description of powdery mildew. 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. Powdery mildews appear as a dusty white to gray coating over leaf surfaces or other plant parts. In most cases this fungal growth can be partially removed by rubbing the leaves so that it might be identified incorrectly as dust accumulated on the leaves. Powdery mildew will begin as discrete circular, powdery white spots. As these spots expand, they will run together producing a continuous matt of mildew which to the casual observer would appear similar to dirt or dust. 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. Yellowing of leaves and death of tissue may result in premature leaf drop. 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. During the winter the fungus is able to survive on infected plant parts and in debris such as fallen leaves. This cycle is generally true for outdoor plants but with houseplants the overwintering stage is of little significance. For indoor plants disease management includes reducing relative humidity around plants. 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. Careful watering techniques will also help. Use trickle irrigation of plants, not wetting the foliage. Also be sure you do not wet foliage after 4 p.m. Give plants breathing room at time of planting. Some success with powdery mildew control is reported, and research being conducted, on "The Baking Soda Formula" using baking soda, horticulture oil, and water. Cornell
Cooperative Extension Yates Association |
