Thank you for your question. Without knowing your location, it is important for me to know what diseases you have in your region, or what local resources you have to address them. We’re your tree in our region, I would suspect anthracnose, described here: https://pnwhandbooks.org/plantdisease/host-disease/dogwood-cornus-spp-anthracnose We do not have the ability to test your plant. I suggest you find a local horticultural agency with a laboratory and staff who can recommend treatment effective and available in your area. Good luck!