This looks like a fungal disease called Cercospora Leaf Spot. This is from the following website: “Circular to angular, small to large, gray or brown spots with dark margins. Some spots may drop out, leaving ragged shot-holes. The spots may be numerous and cause the leaf to turn brown and fall prematurely. Spray applications to control diseases caused by species of Cercospora are rarely, if ever, needed.” And there’s another similar one called Sphaceloma that is described as “Tiny, round to irregular, tan to purplish red or black spots, often with a narrow dark border. The centers may drop out giving leaves a ragged or deformed appearance. Scabby spots may appear on petioles, green stems, flowers, and fruit.” Check out this website (from which we took the disease descriptions), and we recommend you get an accurate ID from the U of Illinois Extension Advisor, a Master Gardener group, or some other professional, so you can determine the best way to deal with the issue, or whether you can leave it alone.
http://web.extension.illinois.edu/forestry/publications/pdf/forest_health/UIUC_Leaf_Spot_Diseases_of_Trees.pdf
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