Powdery mildew is very common on lilac and once you have it difficult to completely get rid of it. Good air circulation is important. After it blooms next year, remove 1/3 of the oldest branches as far down as you can cut it. That might help but isn’t a cure. Clean up any leaves this fall and place in garbage. Spray with an organic fungicide labeled for powdery mildew as soon as the plant begins to leaf out in the spring. Just follow the directions.
The good news is that powdery mildew doesn’t kill lilacs. It’s always worse in dry summers because heavy rains wash some of the mildew spores off – in fact, studies done at Cornell several years back showed that a hard stream of water on the leaves of lilacs once a week was as effective as commercial fungicides! Most people decide to just think of their plants as having gray leaves in late summer, however. If it really bothers you, give the lilac away and plant one of the flowering shrubs that doesn’t get leaf diseases.