Penn State has a publication called Diseases of Peach, Plum, and Cherry. This will give you a general idea of the pests and pathogens that attack plum trees.

To get a more definitive answer, scout all parts of the tree and take samples of the infected tissue. Look under the damaged bark for signs of insect activity. Look at the leaves and around the base of the tree. Collect your samples and have your local Penn State Extension send the samples to the plant clinic for analysis. Penn State will pinpoint the problem and send you an answer along with a recommended treatment plan.