Answer: You may be seeing sooty mold, which is caused indirectly by scale insects. Scales are small, oval insects that suck the plant juices and excrete a liquid called honeydew. The sugar content of this excrement is just high enough to encourage fungal growth. I suspect that's what you're seeing on the leaves of your gardenia. The fungus itself isn't causing harm (though it may reduce photosynthesis somewhat). The underlying problem is the scale. This is a good example of why it is important to determine what is causing the problem before applying any pesticide.
There are several control options. If the plant isn't too large, you can use a gloved hand and squish the adult scales, which look like small brown bumps. Or you can scrub the scales off with a soft brush, then rinse with soapy water. If the plant is too large for this to be feasible, you can apply horticultural oil to suffocate the scale insects. Apply the oil to all surfaces of the plant, and don't spray if the air temperature is below 40F or above 65F. Be sure to read the label, and apply exactly according to directions to avoid damage to your gardenia. Once the reason for the sooty mold has been eliminated, the black coating will weather away.
Q&A Library Searching Tips