We are getting a lot of lilac questions this summer. Most of them are about brown and dying leaves. I am seeing a lot of this in my own neighborhood, especially on common lilacs. The wet spring followed by warm temperature have created the ideal environment this year.And everything has been stressed by the preceding 3 years of drought.

Many are suffering from lilac blight. There are a couple of diseases that cause this. This will not kill your lilac but it is unisightly.

Clean up is essential. Clean up all fallen leaves and dispose of them in your municipal leaf pick up. Do not compost the leaves at home.

http://plantclinic.cornell.edu/factsheets/lilacbacterialblight.pdf

https://extension.oregonstate.edu/news/how-recognize-treat-avoid-lilac-bacterial-blight#main-content

https://hort.extension.wisc.edu/articles/bacterial-blight/