As a result of the thread, I was inspired to research dogwood diseases. While it does appear they're plagued by various diseases, I think the OP may be OK because the trees aren't in a forest setting etc and look to have pretty good air circulation. For my part, I got a bigger appreciation for the tree, and it's ornamental benefits...as it apparently also has awesome Autumn foliage.
I discovered that new dogwoods resistant to the major plagues are available. They're a little expensive but are available online. Meadowbrook Nursery and Girards for example sell the disease-resistant hybrids.
For people who may have their hearts set on a native dogwood but would like to a avoid disease issues, the following website has information about the disease-resistant hybrids:
HGIC 2003 Dogwood Diseases & Insect Pests : Extension : Clemson University : South Carolina