Several fungi cause leaf spots on hackberry. The disease is worse during wet weather but chemical controls are seldom needed.
Witches broom is caused by a mite and powdery mildew, Sphaerotheca phytoptophila. The main symptom is clusters of twigs scattered throughout the tree crown. Prune out the clusters of twigs when practical.
Powdery mildew may coat the leaves white powder. The leaves may be uniformly coated or only in patches. Sprays of benomyl should control the disease.