So here is a picture of what I am facing
Is this a sign of heart rot? What should I do?
It seems to be confined to this one branch (no apples on that branch at all).
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The one good thing no one cared to mention here is that the "fungus" is the Reshi mushroom (The are no "Immitation" variations of Reshi so it's very easy for the trained eye to spot). You could harvest it, dry it, and sell it for up to $40 a pound. May as well get something good out of it, right? But yes, it almost always grows exclusively on decaying wood so, the limb it's on is dead or dying. You can actually take the limb off, let it continue to decay somewhere cool and moist, and let it continue to spore more Reshi.