Sooty mold appropriately describes the disease, as it looks just like chimney soot. Although unsightly, it seldom damages the tree.
The pathogens are dark fungi growing either on the honeydew excreted by sucking insects or on exuded material coming from leaves of certain trees. Sooty mold may occur on any tree but is most commonly seen on boxelder, elm, linden, and maple.
Sooty molds are associated with high temperatures and increased stress brought on by limited moisture. During drought, aphid populations and their honeydew production typically increase on foliage undergoing moisture stress. Keep plants well watered and free of insects.
Sooty molds can be indirectly controlled by reducing populations of sucking insects that excrete honeydew. Use the appropriate recommended chemicals that control aphids and other sucking insects. Also, a good washing of infested tree's foliage (if possible) can dilute the honeydew and wash off mold. That just may be all that is needed.