Thank you, Janet, for contacting the UVM Extension Master Gardener Helpline with your lilac question.

Our office is receiving complaints of browning lilac leaves, as we did last year at this time. The problems are noticeable this year because of the rainy, humid, spring weather we experienced earlier in the season. (Even though we are now almost in drought conditions, we had a very wet spring/early summer season.) After these lilac leaf browning complaints last year, UVM Plant Pathologist, Ann Hazelrigg, prepared a press release, (a copy to that link, Lilac Pests, is attached, below), explaining three common problems occurring with lilacs.

Your lilacs appear to have the last problem explained in this article, called lilac fungal leaf blight. Symptoms include spots, leaf curling, then dropping leaves. As long as the bark looks healthy, and you can see the terminal buds for next year, the plant should be fine.

Ann Hazelrigg's article:

Lilac Pests

Things you should do to help your plants include raking up, and destroying, all of the fallen leaf debris this fall to avoid pathogens overwintering in the soil. Over the winter, while the plant is dormant, thin out the plant, to allow for better air circulation. Thinning will also help to avoid any powdery mildew problems next year. Try to remove the entire branch from the bottom.  Avoid cutting off the tips, of other branches, as you will be cutting off next year's flower buds. (Your lilacs will have already set their buds for next year.)  I have also attached a link from the University of Maine explaining proper thinning/pruning of lilacs.

How do you prune a lilac that has grown out of control? - Cooperative Extension: Garden and Yard - University of Maine Cooperative Extension

I hope you find this information helpful. Hopefully, next year's weather will be more cooperative, and we won't be seeing so many ugly lilac bushes!