Although fungal ID can require microscopic examination, we can say that this is a type of wood decay fungus. In particular, it looks like a species of Pleurotus fungus, but we can't be certain, and with regards to remedies, the exact ID of the fungus doesn't matter much since nothing can be done to remove it. Fungi like these appear only after the tree has already suffered some type of dieback or decline, which could include injury (mower damage, lightning strikes, etc.), root problems, or significant stress. As one of our fungal reference books notes for this type of fungus, "[it] is seldom seen on otherwise healthy trees....when it appears, the tree is usually in an advanced stage of decline."

You can have a certified arborist or licensed tree expert evaluate the tree, but they cannot reverse wood decay or remove the fungus, which is growing extensively in the wood behind the visible mushroom growth (removing the mushrooms will not remove the fungus).

Miri