Our wet spring has caused a few problems this year. We are getting a lot of questions about possible lilac blight. 

I can’t say for sure what kind of blight you have but the treatment is the same. There is nothing to be done this year to treat your shrubs but clean up through the summer and into the fall is essential. 

The effect is mostly cosmetic and will not kill your lilac unless it suffers for several successive years. 

You can send the pruned branches to your local compost. Do not compost  them at home as a small home pile will not get hot enough to kill the pathogens. 

https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/lilac/leavesblotches.html

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