Answer: Lilacs are often affected by powdery mildew by late summer. Although it is somewhat unattractive it does not kill the plant, so generally it is not treated. To reduce the chances of mildew next year, rake up and dispose of the fallen lilac leaves in the trash this fall. Avoid wetting the leaves if you water, and water in the morning rather than at night. Improved air circulation can also help if there are nearby plants crowding it.
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