Black mold or "sooty mold" on plants is a fungal disease caused by honeydew-secreting insects. The insects, usually aphids, scale, whiteflies or mealybugs, secrete a clear, sticky liquid on the plant's foliage. The black mold spores then stick to the honeydew and grow on the plant. To eliminate sooty mold on your plant, you must eliminate the insects at the root of the problem. Between the feeding honeydew insects and the black mold covering the plant, the sooty mold disease can kill the plant if left untreated.
Spray the plant with a stream of water from a garden hose to wash off insects and the honeydew.
Wipe the plant's leaves and stems with a clean, damp sponge to remove all the sticky honeydew and mold growth.
Spray the plant with an appropriate insecticide for the specific honeydew-secreting insect. Follow the application instructions on the label.
Treat the plant with an approved fungicide if the mold returns. You may need to perform more than one application, so follow instructions on the label.
Spray your plant with a dormant oil in late winter or early spring, just before the new growth begins. The dormant oil will help eliminate any over-wintering aphid eggs on the plant.