Good morning Peggy

It is possible that this 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.

We've linked some in-depth info below for you--

“What’s wrong with my lilac? Summer Foliar diseases on lilac” published by the University of Illinois Extension which explains the importance of consistently pruning your bush.

“Leaf Spot on Lilac” published by the University of Missouri Extension 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.

Please reach out again if you need more assistance.