Hello, and thanks for using the Ask an Expert System,
It is difficult to diagnose your problem without actually seeing the tree, and its environment. One issue is that dogwoods prefer some shade. I am guessing, but it looks like your tree is getting full sun. That puts serious stress on your tree giving disease and insects an opportunity to attack. I suggest that you look at A Visual Guide: Problems of Flowering Dogwoods and see if any of those photos match your tree’s symptoms. I think your photos suggest the disease is crown canker. There is no treatment for this disease.
If you have additional questions contact the Lackawanna County Penn State Master Gardener’s Hotline