Hi - Dogwoods in general are susceptible to several fungal diseases such as powdery mildew and spot anthracnose. Your close-up photo of the foliage shows symptoms of both problems which have been very common this year due to the cool, wet spring. We have had a lot of inquiries about these problems on dogwoods from around the state this year since the weather was favorable for fungal disease development.
Here is information about spot anthracnose and powdery mildew.
There is a range of susceptibility among dogwoods to powdery mildew. The best recommendation is to plant powdery mildew-resistant varieties in the future. Pruning for air circulation may help. Clean up and discard fallen foliage in the trash (do not compost it). You will have to monitor the plant in the future and decide if you want to spray fungicides or horticultural oil products listed for powdery mildew. This disease does not kill the plant but can weaken it.
Fungicides will not cure or clear up symptoms that are already present.
At this point, make sure mulch is no thicker than three inches and keep it away from the base of the trunk. Water the tree during dry periods this summer and fall. Prune out any dead wood during dry weather.
Here is more information on care and management of diseases of dogwood.