Hello,

This is black knot disease. Here is the treatment:

“Because the knots are easily visible and localized, removing infected twigs and knots from branches is an important part of disease control. This should be done during the winter dormant period when the knots are easily visible. Cuts should be made several inches (4 to 6 inches) below any visible signs of the knot. 

Cuts can also be made during the growing season if you see knots expanding, but make doubly sure you cut well past any swelling. Burn all infected knotted prunings before the trees break dormancy in the spring. Spores can develop and spread from knots left on the ground or in brush piles.

On large, main branches and trunks, knots should be cut out with a knife or chisel. At least 1 inch of healthy bark around the knot should be removed with these cuts. Taper the cuts to a point at upper and lower ends to promote regrowth of the bark over the wound. If possible, nearby wild plum and cherry trees showing black knots should be destroyed.

Commercial plum growers routinely spray and focus on removing black knots on infected twigs and branches in the winter. Fungicides are applied starting at green cluster and repeated at seven- to 10-day intervals until shoot growth ends around mid-June. Fungicides containing chlorothalonil (FRAC Group M5) are the standard treatment to control this disease. Chlorothalonil sprays are applied during the prebloom and bloom period and also control blossom brown rot. Chlorothalonil cannot be used after shuck split on exposed fruit if that fruit is going to be harvested for human consumption. Chlorothalonil can be used after bloom on ornamental trees where there is no fruit harvest.

Remember that these infections are at least a year old and you will see another round of infection next year from infections that occurred the previous spring. Begin a spring spray program to reduce the spread of the disease. If you put off pruning and spraying, the tree will quickly be covered with knots and will need to be destroyed. Managing black knot in an orchard showing symptoms is at least a two-year project.”

The complete article on black knot is here- https://www.canr.msu.edu/news/controlling_black_knot_in_michigan