It's hard to tell what caused the original damage, though it seems to be either opportunistic wood decay (perhaps set in from a prior injury like periodical cicada egg-laying several summers ago) or a canker infection. In either case, the only recourse is to prune off the damage as it's detected. If the problem persists, we recommend that the tree be evaluated by a certified arborist or licensed tree expert, who can examine the tree in person and look for sources of tree stress that might be able to be alleviated. Arborists can't cure or treat every condition, but they can help to make a diagnosis.

Miri