Answer: It is difficult to keep a plant like this healthy in a container long term under home conditions. It is possible that the soil has become dry -- despite regular watering -- especially if the tree is large in proportion to the container. Unfortunately, based on your description I am not certain if the browning is due to a disease or fungal infection or moisture stress, or possibly a combination since a stressed plant is more susceptible to other problems. I would suggest you consult with your local county extension to try to obtain a more specific diagnosis, in case there is something besides moisture stress. They may appreciate a photo of the overall plant as well as some closeups and/or foliage samples so they can see the pattern of the discoloration and crinkling. IF it is something that requires treatment, they will have the most up to date information on what to use and how/when is best to use it. In the meantime, in case it is a disease, clean up and dispose of any fallen foliage in the trash to reduce sources of reinfection.
If it were mine, I would also plant it in the ground just as soon as possible. Loosen the soil over a wide area and as deep as the rootball to encourage rooting. Water deeply at planting to settle any air pockets, then mulch.
After planting, water (if needed) to keep the soil evenly moist but not sopping wet or saturated until it freezes. To know if you need to water, pull aside the mulch and dig into the soil with your finger. If it is damp, do not water yet. When you water, water slowly and thoroughly so it soaks down deep. After watering, wait a few hours and then dig into the soil to see how far it went -- it can be surprising. It is better to water less often but deeply than to sprinkle lightly every day.
Good luck with your tree -- this is a terrific plant and generally quite pest and disease free.
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