Any idea what this might be? I can't tell if this is animal or disease damage.

This is an asian plum in zone 6.

My guess is fungus or a fungal infection probably where a limb broke off or the plum tree had a wound. I would cut that branch off and throw it away where it will not infect something else. I think the plum tree will be fine but that branch is not. Someone else may have a different idea. Good pictures. Tom

Thanks. I left this tree with six main limbs when I planted it because it was so perfectly shaped. Losing one won't be a big deal, it is already growing faster than I can prune it.