Thank you for contacting the UVM Master Gardener Helpline with your question about your  your lilac bush. The pictures you included were helpful..

We have had a lot of similar questions from people whose lilacs seem to be suffering as well. Following is some information recently provided by the Plant Pathologist.

“There is a fungal leafspot common in lilacs especially in rainy years: https://yardandgarden.extension.iastate.edu/encyclopedia/lilac-pseudocercospora-leaf-spot.
Rake and destroy leaves in the fall and prune to open up the bush to air and light. Fungicides are rarely warranted because buds are all set for next year.”

Here are a few more articles you might find helpful:

. “What’s wrong with my lilac? Summer Foliar diseases on lilac” published by the University of Illinois Extension: https://extension.illinois.edu/blogs/good-growing/2021-08-05-whats-wrong-my-lilac-summer-foliar-diseases-lilac which explains the importance of consistently pruning your bush.

. “Leaf Spot on Lilac” published by the University of Missouri Extension: https://extension.missouri.edu/media/wysiwyg/Extensiondata/Pro/PlantDiagnosticClinic/Docs/leaf-spot-on-lilac.pdf which includes information about managing the disease.

The University of Maine Extension offers this article “Bulletin #2169, Pruning Woody Landscape Plants” which has a link to their video about how to prune your lilac: https://extension.umaine.edu/publications/2169e/

Since you mention you have had this lilac for 40 years, I also want to provide you with information on caring for older lilac trees. 

Lilacs are great, long-lived spring blooming landscape plants. You can do renewal pruning by removing the largest, trunk-like stems and open up younger stems to more light and
air flower. You can also renovate lilacs by cutting the whole shrub down with the understanding you will not have any blooms for a few years.

This is not the right time to prune lilacs if you want flowers next year. Lilacs should be pruned within two weeks of finishing blooming. After 2 weeks, they will set buds for next spring's bloom. So if you wait too long to prune, you'll end up cutting off the flower buds.

We have a good webpage on lilacs which explains more about this spring flowering shrub.

I hope this information is helpful. If you have any additional questions, please feel free to reach out.