If it was mine...I'd smear it with Dr. Farwell's seal & heal...if I didn't have it around, then I'd use another sealer, white silicone perhaps to prevents from water getting in and further rot, [I see a hole]..but most don't agree.

You might want to clean up the roughness with a power tool or chisel first.

Wow that is an old plum tree! How is the flavor on Yellow Egg? Any issues with black knot over the years? Just asking as my Stanley plum got hit hard last year with black knot. Even after pruning out while dormant and spraying this spring it is loaded with fresh cankers. I will cut it down and burn this winter. Just wondering if Yellow Egg had performed well for you .

i'm in the Pacific Northwest and have never had any other problems with this tree. The plums are very large, very tasty, and the tree has been prolific.

uuuhhh is that poison ivy around that tree?

Small blackberry vine...