This looks like a disease called Black Knot of Plum. It is a fungal disease and management relies primarily on pruning out the affected branches. This can be done over several years if there are too many infected branches and you don't want to prune them all out in the same year. Fungicides can also be used to prevent new infections, but they will only be effective if used in conjunction with pruning. Infected cutting should be burned or removed from the property. You can read more about this disease in this article from MSU Extension: https://www.canr.msu.edu/ipm/diseases/black_knot?language_id=