It looks as if it might be some sort of fungus. If you could give me any advice I'd really appreciate your help. It sounds to me like your Dogwood is suffering from a fungal disease called Anthracnose. I'm afraid you are in for an up hill battle. But I have know several badly infected trees that were babied by there gardeners and are now thriving.
Dogwood anthracnose is a fungal disease of leaves and twigs.
The leaves display small to large irregular brown blotches or occasionally purple-edged spots. The blotches are often at the tip of the leaf, triangular, and centered on the main vein. Infected leaves may drop before normal leaf fall. Twigs have sunken spots, which are brown with purple edges. Die-back of infected twigs is common, with dead leaves remaining on the twig.