Hello Bill,

Thanks for your question and yes, canker is the correct term.
I've identified two different stone fruit cankers that it may be, black knot and cytospora canker and I've attached links with more information regarding them both. 

Your photos don't show the gummy ooze that the cytospora canker has, so I suggest it's likely black knot.  Which is a very common disease.  There are management strategies discussed in the attached articles.

Please reach out if you have more questions.

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

https://extension.psu.edu/stone-fruit-disease-black-knot

https://extension.psu.edu/stone-fruit-disease-cytospora-canker