Could be a variety of things. Dogwood Borer is the main pest of Flowering Dogwood. Could be sun-scald from the pic. Could be some type of fungal canker. Without being on site and looking at the tree in person it's hard to say. I get people who send me pictures all the time, and my answer is usually, "when is a good time for me to stop by and take a look". Historic Tree Care is in North Carolina. Not sure if he is close to you. Guy is a bit of an eccentric, but he knows his trees and tree preservation. Give him a call, maybe he can come by and take a look.