Answer: Black Knot is a serious fungal disease and can be difficult to control. The best measures are to plant varieties resistant to the disease and then to be absolutely scrupulous about sanitation: remove and destroy all of the galls while the trees are dormant. This is important because the spores are produced on year old and older galls. (Note that you need to cut several inches past the swelling in order to remove any of the fungus which has spread beyond what you actually see.) You might also wish to contact your County Extension to see if fungicide sprays are recommended in your area, and if so, the recommended applications schedule for your local area. The schedule is important because several carefully timed applications are necessary. While fungicide alone won't cure the problem, it can help in some circumstances. It may also be helpful to remove any wild plums or cherries in the immediate area as they may harbor the disease, too.
I think I would simply remove the plum tree. The disease only affects plums, prunes and cherries.
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