Hello Deb

Shade, high humidity, poor air circulation, leaves frequently wetted by sprinklers, drought stress are all to be avoided to help prevent leaf spot diseases on lilac.

Consider removing 20-30 percent of old branches down to the ground to get better air circulation. Prune back surrounding trees and shrubs that crowd the lilac. Clean up fallen lilac leaves and twigs this fall to reduce overwintering fungi. Next spring and summer apply 1 inch of water per week from below, keeping leaves dry, when rainfall is not expected. Redirect sprinklers so they don’t wet the leaves.

This advice comes from Extension on managing leaf spots on lilac:

“Removing and destroying leaf debris and pruning out dead branches may be the best strategy in reducing diseases in general for shrubs and trees.

Fungicide treatments should not be considered late into the summer. If repeated severe infections occur, preventative spring to early summer fungicide applications may help prevent diseases (but won't eradicate them). Because of this and the fact that lilacs tend to be large and difficult to effectively spray fungicide on, fungicide applications are rarely warranted.”

Also this-

“monitor next spring for the disease. We may not encounter the conditions that favor foliar diseases, which would save you from spraying. Should the lilac display symptoms products containing copper sulfate, sulfur, tebuconazole, or triticonazole would limit the spread of the disease.

“Before you reach for the fungicide to stop the disease, make sure the disease is still active. For instance, spraying for lilac leaf spot now (August) wouldn’t control the disease as the fungus is not currently active. Spray when the disease is active, in the case of our lilacs typically in late spring when the disease first appears.”

References:

https://extension.illinois.edu/blogs/good-growing/2021-08-05-whats-wrong-my-lilac-summer-foliar-diseases-lilac

-https://hortnews.extension.iastate.edu/2021/08/summer-foliar-lilac-diseases

https://hortnews.extension.iastate.edu/how-manage-potential-problems-growing-lilacs

You may email your pictures and the plant history to 

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the MSU Plant and Pest Diagnostic lab for their opinion. Include details you did in your question here. The email consultation is free of charge.

If the lab needs to analyze a branch sample there is a fee for that. Their website describes how to submit samples, and has the lab contact information.

https://www.canr.msu.edu/pestid/

This late in the season you might opt for fall cleanup and pruning. Do not use a fungicide now, as disease fungi are not active now on lilac. Then, if the problem returns next year, submit samples to the lab for diagnosis.