The curling leaves you describe sounds like summer leaf scorch. In hot summer weather, the plant can't pump enough moisture to the canopy as fast as the moisture evaporates from the leaves. Leaf edges and the surfaces between the veins turn crispy brown, curly and contorted. Sometimes entire leaves will die. Let's hope this is the problem, because it's a simple problem that's easily solved. Mulch your tree with 3-4 inches of an organic mulch like shredded cedar or cypress and water deeply once a week during hot dry spells. The other possible cause for your symptoms is dogwood anthracnose. It's a fungal disease that starts as brown splotches on the leaves. Study the splotches closely and you may notice purple edges. Even with the annual application of fungicide, dogwood anthracnose usually kills the tree within a few years. If you'd like help diagnosing the problem; finding out whether it's summer leaf scorch or dogwood anthracnose; take a branch to a local garden center with a Certified Nursery Professional on staff.