Lilac Bush Diseases, Pests
Well-established lilac bushes have grey moldy looking leaves. It does not wipe off. I live in northern Virginia and this season has provided ample but not excessive rain. Also, one bush has leaves beginning to curl and turn yellow. These bushes stand alone, have lots of sun and air. I pruned them lightly after the bloom a few months ago.
Certified GKH Gardening Expert
It sounds like powdery mildew. This article will help: https://www.gardeningknowhow.com/plant-problems/disease/powdery-mildew-homemade-and-organic-remedies.htm