That sounds like the effects of dogwood powdery mildew. 

Dogwood powdery mildew emerged as a problem in our area in the mid-1990's. Caused by Erysiphe pulchra, a fungus that affects only dogwoods, dogwood powdery mildew results in patches of a dusty white coating (fungal mycelium) and stunted, blemished leaves with red-brown lesions. Sometimes the white powdery stage of the disease is unnoticeable. The disease progresses from summer to early fall during periods of humid weather, and warm days with cool nights.

The best management tools are defensive, encouraging air circulation around the tree, picking up leaf litter in the fall and avoiding watering with a hose or sprinkler system that wets the leaves. Already damaged leaves cannot be improved with fungicides, but proactive fungicide application to prevent infection is possible beginning in the spring. It is necessary, however, to reapply the fungicide every few weeks as its effectiveness declines quite quickly and it is an impractical solution for most homeowners. 

Find more about dogwood trees in the home landscape in our Guide to Dogwoods.

Courtesy of NYBG Plant Information Service