Sooty mold is one kind of common fungus that grows on indoor or outdoor plants in certain conditions. It is easy to identify as it makes the plant appear covered in soot, as the name indicates. You should control this mold and take care of the underlying cause of the mold growth for the plants' continued health.
Sooty mold appears as a dusty, dark gray or black mold on plant foliage. On needled plants, it may first look as if the needles have turned black or have been singed in fire. If you touch the moldy area, some mold will rub off on your finger.
Like other molds, sooty mold infects plants via airborne mold spores. But not just any plant may succumb to the sooty mold. This mold is an indication that the indoor plants have an insect problem. According to Cornell University Plant Clinic, an insect secretion, called honeydew, provides sustenance for sooty mold.
Sooty mold can so extensively cover the plants' leaves that they do not get adequate light. This can eventually kill the plants. In addition, even if the mold itself does not quickly kill the plants, the underlying insect infestation may.
Treat sooty mold first by removing it from the plants. You can do this with damp sponges or cloths. After sooty mold has been removed, you'll need to treat the plants with an insecticide to kill insects responsible for honeydew on the plant. These insects include scale, whiteflies, aphids and mealybugs, according to Cornell University Plant Clinic. You can use a commercially available insecticidal soap or insecticidal oil to kill these insects on indoor plants. North Carolina State University Extension advises using an insecticidal oil spray may also help loosen the mold from the plant surface. This may be especially useful if you are unable to remove all the mold by using a damp cloth.
Prevent sooty mold on house plants by keeping the plants free of insects. Check all areas of the plant foliage, especially beneath the leaves and on the stems where insects like to hide, every time you water. Treat the plants for insects as soon as you spot them--this way honeydew will not build up and can't provide food for this mold.