Hello Kim,

Your tree has a disease called black knot. Black knot is a major disease of plum trees in Michigan. It is difficult to control. Read more here:

https://www.canr.msu.edu/news/controlling_black_knot_in_michigan

And here: https://extension.umn.edu/plant-diseases/black-knot#:~:text=The%20black%20knot%20fungus%20overwinters,no%20outward%20symptoms%20of%20disease.

If there are just a few galls, you can prune out the infected branches in late winter—making the cut three to four inches below the gall. Destroy the infected branches.

You may be able to control more infections with a fungicide, applied just before bud break and continued until fruit set. If you use a fungicide, ensure that it can be used against black knot and read the label so that you properly apply it.

You can also consult a certified arborist who can recommend treatment. To find an arborist in your area, go to: www.treesaregood.org the website of the International Society of Arboriculture.

Sadly, if the tree is badly infected, control may be unsuccessful and the tree should be removed and replaced with a black knot resistant tree.

Thank you for contacting Ask Extension.