too much sun or disease

I just purchased a Henry's Garnet Sweetspire as they are supposed to be tough and give color during Fall. I planted it where, in Summer, it gets a good 6 hours of full sun.

The plant is quickly starting to get somewhat even brown spots on the inside of leaf.

Those leaves come off without any resistance. Does it have a disease (don't see any bugs)or is it too much sun.

Thank you for helping to save a young plant.