The standard treatment is to remove the tree from the pot and wash all of the soil off of the roots. Using very sharp scissors remove all of the rotted roots, they should appear brown and mushy. Once that is done the tree should be planted in pure coarse sand, watered once and left alone for at least three years. Water only when it starts to dry out. Do not fertilize until you see evidence of new growth and then only with a low nitrogen fertilizer like somethng you would use for Vegetables. There is no guarantee. A lot depends on how bad the infection and how strong and vigorous the tree is. This is an emergency treatment a sort of desperation solution. Sometimes it works sometimes it does not.

Vance Wood.