My gardenia bushes have a black film on the leaves. I can wipe it off with my fingers. What causes this and how do I get rid of it?
Certified GKH Gardening Expert
This is sooty mold. It happens when the plant gets a pest infestation. I would recommend using neem oil, as this will get rid of the sooty mold fungus and kill the pests. Here is some articles that will help you: