Answer: Based on your description it sounds like the plant has been attacked by powdery mildew which looks a bit like talcum powder on the leaves. It is frequently seen on old fashioned lilacs and will not usually kill the plant although it makes it unsightly. Good air circulation and avoiding wetting the leaves may help prevent it. You could also treat it with a fungicide labeled for mildew on lilacs, but it is not strictly necessary.
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