This is a fungus called sooty mold. It does not infect the leaf tissue, but indicates the presence of piercing-sucking insects such as aphids, white-flies, or scales. As the insects feed, they excrete a sticky fluid called “honeydew” which falls on the leaves below and on which the fungus grows. To stop the sooty mold, you must first control the insects. This can be done with repeated applications of an insecticidal soap, Neem oil, fish oil or a light horticultural oil. Be sure to follow the directions on the spray label and thoroughly spray the upper and lower leaf surfaces as well as the stems and branches.