Thank you, Tammy, for contacting the UVM Extension Master Gardener Helpline with your lilac question, and for providing excellent photos of the problem with the lilac bush.

During the past few weeks, our office has received many reports of the same problem that you are seeing with browning and spotting of lilac leaves, especially in the northwestern counties of Vermont.  The problems are especially noticeable this year because of the rainy, humid weather occurring recently.  Because of this, UVM Plant Pathologist, Ann Hazelrigg, has prepared a press release, (copy attached, below), explaining three common problems occurring this summer.

Your lilacs appear to have the last problem explained in this article, (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 (which are shown in your photos), the plant should be fine.

Things you should do to help your plants include raking up, and destroying, all of the fallen leaf debris this fall.  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 and avoid cutting off the tips, as you will be cutting off next year's flower buds (which are very nicely visible in your photos).  I have also attached a link from the University of Maine explaining proper thinning/pruning of lilacs.

Lilac Pests (uvm.edu)

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

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!