
I am not familiar with this tree, but it looks like it may have some kind of leaf spot disease. Removing the diseased leaves and discarding them far from the garden should help. Or, you may want to contact a local expert, such as an extension agent.
Please see these articles for more information:
http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/diseases/fungal-spots/leaf-spot-shade.aspx
https://www.gardeningknowhow.com/plant-problems/disease/plant-leaf-spots.htm
https://www.gardeningknowhow.com/plant-problems/disease/bacterial-leaf-spot.htm