Hello:

Common lilac (Syringa vulgaris) is susceptible to several diseases including bacterial blight, leaf blight and powdery mildew. See more here: https://extension.psu.edu/lilac-diseases and

http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=m490

In addition, there are several insects that attack lilac such as lilac borer, caterpillars, lilac leaf miner and scale.

Unfortunately, from the picture, it is difficult to see exactly what problem your lilac has. I suggest that you submit a sample to the Plant and Pests Diagnostic Lab at MSU Extension to have the problem analyzed. To learn how to make a submission, go to: https://www.canr.msu.edu/pestid/

You can also contact a certified arborist to diagnose the problem. To find an arborist near you, go to: www.treesaregood.org, the website of the International Society of Arboriculture.

I hope this helps you with you lilac.

Thank you for contacting Ask Extension.