Hi, 

Many lilacs are getting bacterial blight this year. Here's some helpful web pages:

https://yardandgarden.extension.iastate.edu/encyclopedia/lilac-bacterial-blight

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

If you don't think this is blight, this page may help you narrow the cause:

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

The only way to determine if this is blight is to have a sample of the plant tested. The Plant Disease Clinic at the U of M does this, but for a price. You can find more information here:

https://pdc.umn.edu/

Both of the links above suggest ways to manage the disease, including raking up and disposing of fallen leaves and, when it is dry, pruning to open up the plants and allow better air circulation.

That’s all you can do right now. Good luck.

You're welcome!