I have been dealing with this for 2 years and I do have some answers that might be helpful. Although I have not been able to narrow it down to one cause. It is probably because there is more than one reason for it.
First, improper or too much pinching would cause this. Second, possibly heavy root pruning. Third, too much water in a soil that is not draining. Forth, general shedding. I do not think that pesticides or fungicides will do anything to help this issue because it is neither. Although it could be root rot, that is a fungal root issue, not a topical fungus.
Having said this. Baring the situation where this tree was not pinched and it is not shedding season, it is most likely too much water in a soil that is not draining. The soil it is in looks like dirt in the pic. If it were my tree. I would take it out of the pot, comb out all the roots and remove most of the old soil. I would NOT cut any roots, unless they were rotted of course. I would hose off the inside of the pot (just something I do) and repot the tree into good soil. I would give the tree morning sun immediately for a couple of weeks then full sun. As far as winter care, I would not let this tree freeze this winter. Also, if the roots were in bad shape and much manipulation was done. If you have a spot that is in the low 40's to low 50's and gets about 3 hours of sun a day. That might be a good idea.
Also, this can be a very long process. Once this starts, it can go on for many months 3 to 4 months plus. Also, even if you remedy the situation, it will take a while to stabilize. One other thing, if it is a root issue, you will need to do something. It will most likely not stop until the problem is corrected.
I hope this was helpful.
Rob