Hello & thanks for looking!

I've had this Stewartia tree in a pot for a few years, and some of the leaves have recently developed the dried/dead looking patches you can see in the pictures. Between rain and an irrigation system I think it has been receiving enough (maybe too much?) water. Here in New York City we have had some very hot days in the last month, as well as a few days of skies darkened by smoke from the Canadian forest fires. What are some possible causes? What should I be doing differently? I did prune the roots a couple of years ago. I haven't pruned the branches in a long time, and I have never fertilized it.