Yikes! That's a bad one. The term iirc is Black Knot.

I believe all you can do is cut out the Black Knot a few inches below the thing. (Make sure you sterilize the cutter before and after with alcohol or something.) And look up a fungi spray to try to prevent it reoccurring.

I believe all you can do is cut out the Black Knot a few inches below the thing. (Make sure you sterilize the cutter before and after with alcohol or something.) And look up a fungi spray to try to prevent it reoccurring.

Keep the tree growing vigorously and cut that stuff out as soon as it appears deep into clean wood. You use a sharp knife and cut well beyond the galls themselves when you aren't removing an entire branch. When it appears again, cut again. Collect and dispose of infected wood.

I spray the wounds I make with chlorathalinal, which may help. Cornell suggests spraying entire trees with it in early spring as a partial help.

My plum had loads of BK last year, after butchering the tree I sprayed with Lime sulphur and Dormant oil in the fall, and again before bud break. No sign of it this year, but no plums. Should be good next year as I have lots of new clean growth.
Nasty stuff.

Interesting. We get a ton of black knot around here on other Prunus like maydays and Schubert chokecherries, but I've never seen it on my plum trees and I didn't know it could infect plums. When you prune it out, diluting regular laundry bleach with 10 parts water makes an effective sterilant for your cutters between cuts.