Answer: Prunus padus commutata is very susceptible to a disease called black knot, and this would cause the symptoms you have described. In the early stages it can be controlled by pruning out affected branches, cutting off several inches more than the affected part, in early spring before the tree leafs out. In more severe cases a fungicide may also be needed, but it too would be applied very early in the season. Depending on the severity it may require both pruning and treatment with a fungicide to see results in control. Here is a link to a photo of the infection as well as more information about it.
If you need to prune and spray and the tree is at all large, you may want to contact a professionally trained arborist with experience working on ornamental trees to help you. Good luck with your tree.
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