We have a very old apple tree with four trunks coming off the main trunk - last weekend's snow here took down one of these trunks near the base. Turns out it was riddled with a white spongy substance (see pictures). We had dead wood along the top, fungus(?) working in from the surface and white fungus(?) through the wood. Questions are: What is it? Is it treatable? Is it likely to be local to this branch, or is it most likely systemic – is it likely I’ll lose the whole tree?