Thank you for your question and pictures for Extension. It appears your tree lilac has a leaf spot disease caused by a fungus. We have had an abnormally wet spring/summer and many shrubs and flowers are affected by leaf spot. According to the article below, you do not need to do anything unless this occurs several years in a row. It's also important to rake up and dispose of the leaves in the fall: 

Leaf spot diseases of trees and shrubs | UMN Extension

Here is a fact sheet on Japanese Tree Lilac from the Missouri Botanical Garden:

Syringa reticulata - Plant Finder (missouribotanicalgarden.org)

Wishing you best of luck with your beautiful tree.