Answer: It's possible your gardenia has an infestation of scale insects (or some other sucking insect). When insects suck the plant's juices, they excrete what is called 'honeydew', a sticky substance that has a high enough sugar content to encourage the growth of mold. Inspect the stems and undersides of the leaves of your plant to identify the critter that's causing the problem. If you can squish the scale insects, you can eliminate the honeydew and subsequent sooty mold. No treatment is required for the sooty mold. The mold on your leaves will eventually wash away with rains, or will crack and fall off on its own.
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