Rather than constantly repeating my mantra on this cruddy tree please read previous posts with SK in the subject line...that should answer all your questions...This tree should be banned for causing stress and eventual mental illness for so many unsuspecting buyers (except in the PNW)!!!! David

First of all is the tree in alot of sun? If so, this is very common as these trees are not very sun tolerant in my experience and the exact same thing has happened to 30 or so sango kaku specimens at the nursery (in fact they started doing it as early as late june but they were in full sun and potted which amplifies the situation). Even if in some shade this heat wave we've had (100 F and above for 2 weeks), has partially defoliated almost all of our stock even when under 30 percent shade cloth. This is common stress due to heat that acer palmatum is not used to since japan has a relatively mild summer.

I doubt it will die but they are weak trees IMO and are really not very easy to grow to be as common as they are. Make sure you are keeping the soil lightly moist (not wet), and if things get too bad you may have to move it to a more shady location. Other than that you just have to wait it out until the root system is more established in a few years. When this happens you may see some yellowing in the hottest part of summer but hopefully not browning or defoliation.

Most likely the rootball is too dry; and that can occur in spite of rainfall and watering. To know for certain, check it.