I'm spending my first Midwestern summer with a mature cherry tree in the yard of my new house. Up until now everything has been going great with this tree (had a good crop of cherries, vigorous leaf growth) but I've noticed in the last couple of weeks that many of the leaves have fallen off. This has coincided with very hot and humid weather in our area of the country (Zone 6b). This is the first cherry tree I've taken care of so I'm wondering if this leaf fall could be due to heat stress or something even more serious/deadly. I don't want to lose my tree! Thanks for your help!

I've attached some photos to illustrate the problem. The first photo shows the brown, dead leaves on the ground. The second and third are closeups of other stricken leaves on the tree.