In many parts of the US it has been a wetter spring then usual.
This can cause leaf spot diseases. Aphids can also cause
yellowing of leaves. From this site: Sunken Spots On Leaves
Anthracnose - This fungus disease causes distinct lesions on
maple leaves, which appear as moist, sunken spots with fruiting
bodies in the center. The leaf spots may run together, causing a
blotch or blight. The dead areas follow the veins or are bounded
by larger veins. Sometimes the ends of young shoots blight down
to several inches. Pustules containing pinkish spores appear.
Dieback and loss of foliage may occur in severe cases. Gather
and destroy diseased leaves when they fall to prevent the spread
of the fungus. Replace existing mulch with fresh that does not
have fungal spores on it. Prune away diseased branches, then
disinfect your pruning saw or loppers by dipping them into a
solution of hot water and household bleach. Maintain tree vigor
by feeding and watering it well, especially during droughts.
Mature trees can usually