I believe you've correctly caracterized it as a fungus in your own assesment. Which one is not clear from this photo.
What may be less obvious if you haven't looked at fungus behavior is that it's the "fruiting stage" that you see, which will eventually produce spores, and it's been invisibly feeding on a part of the tree that's already dead to get to this point, so the prospects for the tree may not be very good.
I am familiar with some cases where a living tree with a dead heart (which eventually becomes hollow) has tolerated a fungus eating the dead parts for many years without expiring. Importantly, this is highly unlikely to be similar to a fungal disease like powdery mildew that directly attacks a tree - nearly all the wood-eating fungi are opportunistic exploiters of wood that's already dead (not killed by the fungus inhabiting it.) So killing the fungus won't do much for the health of the tree, which was already in trouble.
The fungus won't be killed by winter, though the visible evidence of it may fade away until next year around this time.