Dog Wood Tree
Based on your photos this looks like old damage from powdery mildew, a fungal disease. Now all you can do is rake fallen leaves to prevent overwintering spores. 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. http://extension.umd.edu/hgic/powdery-mildew-trees-and-shrubs See our publication on management and control.