I would suspect water as well. I live in VA and keep mine outside year around. Mine get water 2 times a day on an automatic system. If you let them get dry they suffer. If you noticed the tree wilted one day its defiantly a water problem. For soil I use a combo potting soil or compost mixed with turface, akadama and pumice. It drains but has enough material to keep the plant wet at all times. As for sun they like partial shade. Mine this time of year get allot of sun no leaves on my big trees in the yard. The leaves will change color from a solid green to more reddish brownish if they are getting to much sun. More noticeable if you turn the plant from the shady side to the sunny side.
