Growth on plum tree
This a fungus known as black knot. You should prune it, and any others you find, off the tree while the tree is dormant- right now is a good time to prune. Make the cut about 6 inches below the knot into the healthy part of the branch. Take the pruned sections and bury, or you may burn it if you are allowed to burn debris in your area. Do not leave it on the ground, because this spreads more fungal spores in the area. Examine the surrounding area for wild cherry, cherry, plums or other trees that show these knots and remove them, too.
Here are details, with recommended resistant trees, and fungicides you can use after the knots are removed. Fungicides only prevent new knots when applied at the correct time, they do not cure existing knots.
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