Answer: What you describe sounds like a fungal disease called powdery mildew. Powdery mildews, as the name implies, often appear as a superficial white or gray powdery growth of fungus over the surface of leaves, stems, flowers or fruit of affected plants. These patches may enlarge until they cover the entire leaf on one or both sides. Young foliage and shoots may be particularly susceptible. Leaf curling and twisting may be noted before the fungus is evident. Severe powdery mildew infection will result in yellowed leaves, dried and brown leaves and disfigured shoots and flowers. Although it usually is not a fatal disease, powdery mildew may hasten plant defoliation and fall dormancy and the infected plant may become extremely unsightly.
Most powdery mildew fungi produce airborne spores and infect plants when temperatures are moderate (60 to 80 degrees F) and will not be present during the hottest days of the summer. Unlike most other fungi that infect plants, powdery mildew fungi do not require free water on the plant surface in order to germinate and infect. Some powdery mildew fungi, especially those on rose, apple and cherry are favored by high humidities. Overcrowding and shading will keep plants cool and promote higher humidity, both highly conducive to powdery mildew development.
In many cases, powdery mildew diseases do little damage to overall plant health, and yearly infections can be ignored if unsightliness is not a major concern. Lilacs, for example, can support powdery mildew every year with little or no apparent effect on plant health. On some plants, however, powdery mildews can result in significant damage and fungicides may be necessary to achieve acceptable control. For best results with fungicides, spray programs must begin as soon as mildews are detected. Spray on a regular schedule, spraying more often during cool, damp weather. Use a good spreader-sticker with the fungicides. Be sure and cover both surfaces of all leaves with the spray. Use only fungicides registered for powdery mildew control, such as Banner, Bayleton or Funginex.
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