Answer: These trees are difficult to overwinter successfully even in large containers. The winter stress (not only cold on the roots but also especially spring freeze thaw cycles) then accumulates along with the energy expended for the spring growth spurt and it is sometimes just too much. There might also be a fungal or other problem happening, often weakened plants are affected by other problems, too. It is difficult to diagnose this type of problem long distance, so I would suggest you work with your local county extension and/or a professionally trained and certified nurserymany to try to see if it can be saved. I'm sorry about your little tree.
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