Asked July 16, 2018, 8:38 AM EDT
Oakland County Michigan
The most likely issue is lack of moisture. Dig down carefully about 8 inches in the root zone and see if the soil is moist. Too little, or too much, water will cause early leaf drop. Give a deep, slow watering all the way around the tree. Use a soaker hose or a sprinkler set very low, so that it just trickles, moved several times. This can take 2-3 hours to do. Don’t water to the point that water is puddling on the soil surface.
There are some leaf spots caused by fungi or insects. If you can attach a clear picture of the whole tree, and one or two of a couple freshly fallen leaves ( one turned so the back can be seen) I may be able to be more specific. ( we can magnify the pics so go for clear shots, not so close as to be blurry)
Keep the soil in the root zone moist down to a depth of 8-10 inches, but not soaking, especially now during hot weather. Rake up fallen leaves and discard. Trees can lose up to 1/3 of their leaves and still be OK, if given enough water. Consider placing a 2-3 inch layer of chipped or shredded bark around the tree out to where the farthest branches reach, but pulled back and not touching the trunk. Monitor the branches and leaves for insects or unusual spots on bark or leaves.
Here is a reference for you -
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