Based upon what we can see in the photos, the leaves appear to be affected by powdery mildew as well as a leaf spot disease.

Lilacs in the Twin Cities area have been plagued by leaf spot diseases in recent years.  The pathogens are often difficult to identify without laboratory analysis.  

https://www.startribune.com/what-s-wrong-with-the-lilacs-beloved-plants-in-minnesota-threatened-by-leaf-fungus/572649202/

This problem hasn't been limited to Minnesota:

https://extension.missouri.edu/media/wysiwyg/Extensiondata/Pro/PlantDiagnosticClinic/Docs/leaf-spot-on-lilac.pdf

https://yardandgarden.extension.iastate.edu/article/2021/08/summer-foliar-lilac-diseases  

If questions remain after you have read the publications, consider sending samples of affected leaves to the University of Minnesota Plant Disease Clinic.

https://pdc.umn.edu/

Fortunately, lilacs are resilient.  

Following are comments from Illinois Extension:

What can be done?

Yet, all hope is not lost. Many of these lilacs will recover next year even though flowering may be affected.

A common thread with most affected lilac samples I’ve received is that they were older varieties, poorly pruned, or not pruned at all. Pruning is critical to reducing disease opportunities in multi-stemmed shrubs like lilac. When it comes to pruning lilacs, put the hedge shears down. Instead, reach for loppers or a handsaw and remove a third of the thickest (oldest) stems at the base of the shrub, near the ground. This is called renewal pruning and can be done annually or every few years. Renewal pruning opens up the shrub, promotes better airflow, and gives younger growth the room to develop and put on a great flower show.

If a lab test confirms you're dealing with the fungal Pseudocercospora leaf spot, monitor next spring for the disease. We may not encounter the conditions that favor foliar diseases, which would save you from spraying. Should the lilac display symptoms products containing copper sulfate, sulfur, tebuconazole, or triticonazole would limit the spread of the disease.

Much of the advice in the above paragraph rings true for the bacterial disease Pseudomonas spp. However, for chemical control, the active ingredient copper octanoate is recommended.

If the disease reoccurs every year, it may be more cost-effective to replace the lilac with a newer variety that has been bred to resist these common diseases.