We have had many questions in Minnesota this year regarding lilac leaf diseases, especially from the southern part of the state. If you search old questions, I'm sure you can find a few answers by me and other Master Gardeners.

We cannot identify these diseases from photos. For one thing, many of them have similar symptoms. Here are some links to information on a few of these diseases:
https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs#bacterial-leaf-spots-and-blight-1156513
https://yardandgarden.extension.iastate.edu/encyclopedia/lilac-bacterial-blight
https://yardandgarden.extension.iastate.edu/article/2021/08/summer-foliar-lilac-diseases

If you want to identify exactly what your lilac has, send a sample to the plant disease clinic:
https://pdc.umn.edu/

However, in most cases, your lilac will survive, although bloom may be reduced next season. And some simple disease management tricks (careful fall cleanup, water management, good air circulation, etc.) will help in the future:
https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs

Many of the lilacs we've been asked about are quite old. Older lilacs can often benefit greatly from renewal pruning (remove 1/3 of the old wood each year for three years). You might consider starting such a program next winter:
https://www.extension.iastate.edu/news/2006/jan/072602.htm