Wilting leaves on a lilac could be caused by an insect infestation or too much sun or not enough water or fertilizer applied at the wrong time or bacterial blight. I suspect bacterial blight since bacterial blight seems to be widespread in the Upper Peninsula. Lilacs should receive at least an inch of water per week and an application of fertilizer each Spring. The best way to treat bacterial blight is by removing the infected branch or branches and disposing of the branches by burning or placing them in a land fill. Do not compost the branches. Trim the infected branches down until you can see a distinct green ring just underneath the bark, between each cut dip your pruner into a bleach solution. This will help prevent spreading the bacteria to a new location. A preventative spray with a copper-based fungicide may help to control bacterial blight.

Also clean up last year’s leaves from the base of the Lilac. The old leaves may winter over fungal spores that could affect the Lilac this Spring. Make sure to water and fertilize the Lilac once the buds begin to break in the Spring.

The following web site provides more information on lilacs and bacterial blight.

How to recognize, treat and avoid lilac bacterial blight | OSU Extension Service (oregonstate.edu)

Bacterial blight of lilac - MSU Extension

https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs

Thank you for your question and have a great gardening experience.