It's called black knot.

a recent post: https://www.houzz.com/discussions/5726060/does-anyone-know-what-these-nest-or-poop-things-are-on-my-tree-branche#n=9

its the same possibility with production plums.. its why i quit growing stanley plums .. as well as the purple leaf plum

ken

Thanks. Will it go away next season or is plum tree done?

Green Gage plum tree took 9 years to produce a few plums then gets infected. Disgusting.

I have a peach tree nearby, will it spread to it?

No, it will not go away. It is a permanent fixture at this point and can eventually weaken the tree to the point of it dying. It could spread to the peach but not that likely. It is primarily associated with plums and cherries and rarely with any other stone fruits.