Sounds like typical trouble with this troublesome tree I suspect it is a Sango kaku. I think your tree is history and there was probably NOTHING you did wrong ...these trees seem to be disease prone and suseptable to both cold die back and heat damage and should only be planted in "perfect" JM growing areas in this country...IE:the North West.
I personally wouldn't waist alot of time with it ...you will probably have a nice root stock (if it is grafted) come up next spring so you'll at least have a green or red generic JM .. If you dig it up and plant another JM I suggest you NOT plant that particulsr cultivar and not plant in that exact spot since it may have diseased up your soil. David

OOPS I didn't read the subject but that was obvious it was a Sango kaku ... anyway the above is my take on that cultivar even though I just guessed since there are several red branched cultivars that one is the most popular and I've heard it a bizzillion times from just about everyone who has bought one!!! but I should have read closer the subject subject line ;>) David

Well it might not be dead yet...but it doesn't sound good. Most of our stock at the nursery (not just sango kaku) are either partially defoliated or in some cases completely defoliated. However, we do not have black dieback (although some sango kakus have shown some dieback). But I agree w/ david it isn't a tough tree by any means and should only be sited in partial shade in our area IMO. Nevertheless keep watering and good luck with it...all you can do when it gets over 100 for a sustained period like this is cross your fingers.

Though not in the perfect zone, I did plant in the perfect spot. Dappled sunlight, well watered and well drained. The only thing I did not do was fertilize.

The black die back is only about 6 inches down into the 4 feet worth of trunk. There are still live branches and leaves. Should I cut back to the point of living tree, or not add this insult and wait it out?

Rob

You might want to post a couple of photos if you can figure out how ...but even w/o you can certainly try to off any dead or dieing branches and cut it down past any black area(s).....you could also try to treat the black trunk areas with fungicide... which might be a better idea for any areas that are an integral parts of the tree (ie: the trunk) since you don't want to wack it so bad it dies anyway ... It really depends on how much effort the tree is worth to you it certaily won't hurt to try a bit ... make sure to sterilize the clippers after every cut ( I use hand sanitizer) and after the last cut. I personally wouldn't bother but don't want my intense dislike for this tree to taint your actions...or perception of possible success in saving it ...which in all honesty is unlikely but I assume possible ...David