Dogwoods in general are susceptible to several fungal diseases such spot anthracnose and powdery mildew. Your photos of the foliage show some leaf spot and powdery mildew which has been common due to our cool wet spring.
Spot anthracnose (leaf spot) is not usually a big problem, but we're seeing a lot of it this year because weather conditions were perfect for it. Since disease severity is sporadic depending on how wet the spring season is routine fungicide applications are not recommended.
There is a range of susceptibility among dogwood to powdery mildew. The best recommendation is to plant powdery mildew resistant varieties in the future. Pruning for air circulation may help. 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.
At this point, make sure mulch is no thicker than several inches and keep it away from the base of the trunk. Water the tree during dry periods. Prune any dead wood.
Here is more on care and management of disease of dogwood. Look for resistant varieties in the future.