Hi:
Did the tree have mulch and/or dirt higher up against the trunk?  Dogwoods usually suffer from dogwood borer (a clearwinged moth whose larvae feed inside the tree) and dogwood anthracnose (a fungal disease).  They are an understory tree (filtered sunlight) and growing in full sun surrounded by turfgrass usually cause it to decline.  I can't tell exactly what caused this injury.  It looks like an old wound.  You can send more pictures showing the environment and canopy.