Gardenia Black Scale
I have a very large gardenia bush outside my window. For the past 2 years, it has been covered with a black substance that is now all over the wrought iron fence below it. The plant APPEARS to be healthy, i. e. no yellow leaves, but this black covering is unsightly and I'm sure not good for the plant. Is there anything I can do, short of cutting it down?
This sounds like sooty mold, which is a fungal disease which affect gardenias.
It is treatable. Please see this article for further information: