Apparently there is no successful treatment for this fungus, which attacks exposed wood -so there may have been some damage to the bark to give it entry initially. Sadly it does seriously weaken the tree. However in the meantime you might like to make use of its medicinal properties - plenty of info on this on line.

Hi Jules,
This fungus usually grows on dead wood, whether fallen or still standing. Was your tree ok last year, or could it have been dying already? I don't know of a cure and would be inclined to sacrifice the tree. Sorry to be negative about it, however, let's hope one of the tree experts on here has a more positive solution for you.

I can't give much hope myself. Turkey tail fungus is a symptom, rather than a cause, since it only attacks dead wood, as Pennyfarthing says. Growth of the fruiting bodies that extensive is a sign that the tree is pretty far gone, and will be difficult to save, if possible at all. Even if you restore it to apparent health, fruiting bodies will continue to appear, the center will continue to rot, and the hollow tree will eventually fall.

Wow, that's some infestation. It also looks like you might have removed quite a lot of the tree in the past? Which would explain how disease got in there. So sorry...looks like you will lose it. My friend lost a small cherry tree with bacterial canker a couple of years ago. Disease happens. :(