What is that black stuff on the leaves of my gardenia (or citrus, shrubs, etc.)?
A. That’s likely to be sooty mold — a black, powdery-looking substance that grows on the secretion of insects (known as “honeydew”). This secretion can be left by aphids, mealy bugs or scale and the cure is to control the insect. If you don’t see the insect on the affected plant, take a look above the plant as it could be that the insects are present on a plant above. Treat the infested plant with a horticultural oil spray, such as All-Season Oil Spray or Organacide or Insectidical Soap.
