Zog that "rot" looks a lot like staining from embedded metal from what I see on my little phone. Let us know what you find when you split it.
It's stained plus rot. The tree finished croaking this summer, last leaves went and the little branches fell off. I didn't have the detector with me today but will tote it down and scan the chunks and the stumps when I go to move the rounds. I know I didn't hit any foreign objects.
Most of the old fenceline trees here show obvious external markings from old fence, easy enough to see, but none of these trees down that row have any indication of that, so..I dunno.
There's a LOT of junk in the ground around here, real old farmland everywhere, plus, civil war battlefield.
I have yet to metal detect anything cool though, just old garbage and trash, this farm has been milked dry over the last several decades by pickers. They got all the good stuff.
Jusging by the location of that discoloration, the tree grew right over something metallic, but I ain't digging up that stump, either.