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

I asked the leader of the Plant Diagnostic Clinic to take a look at your pictures to help us identify the issue. Here is her response:

“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.”

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/

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