Hi Cliff,
   It is a leaf spot fungal disease, possibly cercospora. Recommend removing some of the leaves that are affected and disposing of them.  To the best of your ability spraying Neem oil as instructed on the label and doing this will help keep this in check.  Doing this regularly this fall should improve the environment so the tree is healthier next year.   Here is a good article from Extension at the University of Minnesota that gives thorough overview in the prevention& treatment of leaf spot.
https://extension.umn.edu/plant-diseases/leaf-spot-diseases-trees-and-shrubs#identification-1156261
   Wishing you all the best!
Bob Kunkel