I apologize for this delayed reply to your gardening question and hope it is still helpful to you.
The black sooty mold on your gardenia leaves is caused by whiteflies, scale, or some other small insect which produces a sticky substance on which the mold and then grows. If you can identify and control the insects you will be stopping the mold as well. Consider taking some leaves and shoots to your County Extension Office for them to try and identify the insects causing the problem. They can then recommend an appropriate control depending on what they find. In the absence of this diagnosis visit your Home Depot garden center and look for a product with an ingredient called imidocloprid.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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