Thank you for your question. Extension is getting multiple question about this same lilac problem. Here is information for you:

Your lilac is experiencing a blight condition. This is now extremely common in the Metro area. It could be a blight caused by the bacterium Pseudomonas syringae. Among other things, it is characterized by leaf spots that can lead to leaf darkening, leaf wilting, and premature leaf dropping. Fortunately, this is largely a cosmetic condition and unless it occurs over several seasons, there are no adverse effects. It is associated with wet conditions, which certainly describes our current 2024 season. Because this is NOT a fungal pathogen, application of fungicides will do no good. It can be transmitted from one season to the next via leaf debris. To prevent transmission from one season to the next, thorough fall cleanup is essential. See the following for further information and remedial procedures:

https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs - bacterial-leaf-spots-and-blight-1156513

It really is not important which type of blight exists; it should not be a fatal situation for your lilacs. Do not feel that they must be dug up immediately and discarded. They will recover although their blooming may be delayed by a season or two.