I don't want you to ahve to wait any longer, so I'm going to go ahead and give you an answer. The additional information may have helped to narrow down the causes, but from your photo, it looks like the lilac is suffering from what is called leaf curl. Leaf curl can be caused by a number of conditions and pests, but when it occurs in mid to late summer or early fall, it is often what is called leaf curl necrosis. it is due to air pollution. Lilacs of all types are very sensitive to air quality. Exhaust from cars on the street, pollutants from nearby factories and power plants, and fumes from lawn chemicals used in the neighborhood, are common causes. Even excessive ozone in the air can cause it. Unfortunately, there isn't anything you can do about this except to give the lilac optimal growing conditions, with the right amount of water and fertilizer. Often, the plant will deteriorate to the point that it dies. If the entire upper part of the lilac is dead (you can tell by scratching off a bit of bark to see if the wood is green or if it is dry and white or brown), new growth may come up from the roots the following spring.
More rarely, insect pests can cause leaf roll, but you should be able to see signs of them. Look for insects themselves and webs on the undersides of leaves. If you find signs of insects, you can treat with an insecticidal soap from a garden center or nursery. If you'd like to have someone examine the leaves to be sure of the diagnosis, I recommend your County Extension Agent. This is a government program through universities, and there is no charge for the service. They won't provide any treatment, but if you take them a sample, they will help with diagnosis. To find your Agent, click on this link, then scroll to your county:
I wish I could give you better news, but I believe you deserve honesty. It's very likely that your lilacs are suffering from leaf roll necrosis. If you have more questions, let me know by clicking on REPLY.
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