Thank you!

Dogwood trees are often affected by a fungal disease called Powdery Mildew. The Powdery Mildew fungus causes the leaves to have a light grey or white substance on them. This sounds very similar to what you have described affecting your dogwood trees.

The recommended treatment for this fungus is using any one of various fungicides.

"The North Carolina State University Extension recommends several sprays throughout the growing season, including as flowers and then leaves emerge, during the middle of the summer and around the time the leaves begin to drop. Some of the chemicals to use when spraying a dogwood for powdery mildew include azoxystrobin, propiconazole and myclobutanil, according to the North Carolina State University Extension".

It also recommended to remove any fallen leaves or branches and to prune the trees annually. This is meant to control the presence of the Powdery Mildew and to prevent it from spreading further. Be sure also that moisture drains away from the trees to prevent additional fungal growth on the tree.

I recommend speaking to a local arborist about the concentrations and proper ways to treat the trees with fungicides. If you are not comfortable doing this on your own, it would worth a phone call to a local company that specializes in the handling of these fungicides.

Feel free to post any follow-up questions you may have.