Several fungi cause cankers leading to dieback. Fertilize to keep plants healthy and prune out infected parts.
Fusarium wilt infects roots, causing the leaves to droop and wilt. A light infection causes only gradual dwarfing or yellowing and premature red leaf coloration. There is no chemical control.
Leaf spot caused by Pezizella oenotherae causes gray spots with purplish margins that merge, giving the leaves a scorched appearance. No chemical control is listed.
Various genera of powdery mildew forms a white coating on the leaves. Use sprays of benomyl or sulfur.
Verticillium wilt causes wilting of individual stems, followed by death of the foliage. Eventually the entire plant dies. There is no chemical control for the disease. Prune out infected branches. Do not replant in the same spot with sumac or other susceptible plants.