You are likely seeing one the hundreds (thousands?) of fungal decay organisms that are found worldwide. The fungi feed on decaying organic matter and on your dogwood that is the dead tissue of the bark and below. There is no effective treatment that will control the fungus that I am aware of. Cleaning up the wounded area, removing any decaying bark back to the healthy edges of the wound should be done. If he wound was caused by a severe cold winter, wrapping the trunk to prevent future frost cracks should be done. Lastly, your more local Extension Service may be able to provide you with a more focused solution.
