Maple Leaf SpotsAdvice from the City Forester City of Chicago Streets and Sanitation, Bureau of Forestry about the white spots appearing on maple leaves:
Powdery mildew on the leaves is common throughout Chicago and not anything to worry about. Usually this fungus is brought on by environmental conditions being just right. Some years we don’t see much of the fungus, although the pathogen which causes it is always there. Also, it tends to pop up in different areas and recurs for a few years and then disappears. But long story short, it will not cause any long term damage to your trees.
One thing homeowners can do to minimize this disease, is to be sure to clean up all of the fallen leaves at the end of the year. Other than that, try and reduce any possible environmental stresses upon the tree. If you have compacted soil in your parkway you might consider renting a soil aerator and aerating the soil. Also avoid building any planters around the base of the tree. Although this practice seems to beautify the neighborhood, it really can be very damaging to the tree, specifically the tree roots and base of the trunk. This will almost always cause a long lingering death for the tree sometimes over 5 -8 years, depending upon the extent of the damage and usually the resident doesn’t realize this is killing the tree.