From the ArcaMax Publishing, Gardening Newsletter:
http://www.arcamax.com/news/gardening/s-2540-457094
Gardenias can be infected with a fungus called sooty mold. This black mold grows on the sweet honeydew produced by the feeding of sucking insects such as aphids. Control the insects with insecticidal soap to control the mold.
This news arrived on: 06/26/2008