Pick and destroy affected leaves before the lower surface breaks, freeing those spores to build new lives. Fungicides can be found. Dogwood spot anthracnose is a fungus that attacks flower bracts, leaves, young shoots and fruit, in that order. You'll see small, reddish-purple round or elongated leaf spots about the size of a pin head. Spots have yellow centers and brown or black margins.
Centers finally drop out.
Usually, damage isn't really horrible. Prevention can be aided by thinning the tree's canopy to promote better air circulation. There are fungicides and tolerant cultivars.