Your shrub probably has bacterial leaf spot and powdery mildew.  Both are common on lilacs in Minnesota.  Good air circulation will help to prevent this in the future. It is essential that you clean up all fallen leaves and dispose of then in the garbage.  Home composting does not get hot enough to destroy the fungi.  If you haven't done any rejuvenating pruning you may want to consider it.  Cut 1/3rd of the oldest thickest stems to the ground.  This will encourage new young stems which also will bloom more than the old ones.  This also allows for more air circulation.  Do this three years in a row and your lilac will be like new.  Click on the drop down links in this page: Leaf spot diseases of trees and shrubs | UMN Extension