I wouldn't prune anything yet. Adverse conditions can cause a loss of foliage but there is the potential for it to releaf. If there are no new buds forming on that branch in late fall, then it is likely dead and you can safely cut it off.

Is this in full sun? I would consider that a less than ideal situation for this maple as it is prone to leaf scorch, as are most JM's with very light colored or gold foliage. Even though my garden has nowhere that is full sun, I moved mine to a much shadier location as it was showing signs of sun stress. It now only gets sun until about noon then dappled shade for the rest of the day.

If under a guarantee, I'd consider replacing. And re-siting. Or select a variety that will hold up better to lots of sun.

Unfortunately, I specifically picked the "autumn moon", because the attached nursery information claimed it could handle direct Sun (Southern exposure). As I mentioned, I'm in a very moderate climate, near San Francisco. If it can't handle the sun here, I doubt it can handle it anywhere!

Have you ever watered it?

I was afraid that might be the answer. There is a small JM up the street with the same exposure that seems to be doing well, but it's the dark variety. As a neophyte gardner, the contradictions I get from different nursery staff and especially nursery literature is very frustrating.

Which brings up Mike's point about watering. Yes...I've paid strict attention to watering. However depending who I talked to last, sometimes I think I'm overwatering. I talked to two guys from the same nursery (a real nursery, not Home Depot). One guy says "stick your finger down into the soil. If it's cold and moist, you don't need to water". Another guy (who I've actually seen doing Bonsai cuttings) went off on a 5 minute spiel about how the soil may be dry at the surface, but the roots may have moisture available below, and should be encouraged to go down for it. My only guess is that the first guy was thinking (appropriately) about a newly planted tree, while the second guy was thinking of an established one. Yet a third guy thought that the die off was probably do to the late spring rain we received, and I think "Am I killing this poor thing with kindness?". So, I leave the nursery shaking my head, more confused then when I arrived.

I've always wanted a JM but thought the exposure and my skill level probably forbade it. I think I should have stuck with that wisdom!

A slow, deep soaking a couple of times a week - if that - should be fine. And by slow, I mean leaving the hose on at a slow trickle and letting the water soak well down past the root ball. As long as your soil drains well, spring rains should not be an issue and I doubt had any impact....it rains pretty good here in spring as well :-) But good drainage is key - JM's will not tolerate soggy soil conditions.

If you want to replant in that location, I'd consider something like Fireglow. Most red leaved JM's will tolerate a lot of sun and FG in my climate does very well in full sun. And it will look stunning backlit by the southern sun......it really lives up to its name!!