Answer: This late in the growing season I suspect your lilac bush has developed powdery mildew. This often happens in the fall when daytime temperatures are warm and nights are cool. Lack of sufficient sunshine and poor air circulation makes lilacs more susceptible to powdery mildew. After the leaves fall be sure to rake them up and remove them from the garden. Next spring, after your plant has finished flowering, prune it to increase air circulation.
Best wishes with your lilac bush!
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