Thanks everybody, for the input. I'll hold off on the fertilizer, water deep, and put a better shade lattice over it. Dennis, I doubt I'll see another drop of rainwater until mid-July, but our neighborhood is supplied by a well and the quality is quite good.
Dan, I think your observation about high heat near the ground is right on the money. This little tree sat out in full sun through the hottest part of the summer last year and loved it, but it was up on a rack about a foot off the ground, and the black pot was sleeved inside a white bucket to keep the roots from overheating. In addition, all the pots sat in shallow water basins, so they could keep a little cooler just through transpiration. It might have escaped damage this year with increased irrigation, but it's hard to say--it's the shortest of all the others, by far, and I think the combination of direct sun and reflected heat off the mulch layer has created a shallow "oven zone" that's too much for any leaves growing right near the surface. I looked at the other trees once again, closely, and on the Conadria noticed similar damage to part of one leaf that is very close to the ground. Maybe I'd better put shade over some of the others, too, just to be on the safe side.