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 sticky fluid called “honeydew” which falls on the leaves below and upon which the fungus grows. To stop the sooty mold, you must first remove the insects. This can be done with repeated applications of a fine, horticultural oil. Be sure to thoroughly spray the upper and lower leaf surfaces.

Additional Resources

http://edis.ifas.ufl.edu/HS141