The powdery coating on house plants that is often called "black dust" or "soot" is sooty mold that grows on the sugary honeydew secreted by insects that suck sap from plants. Sooty mold often appears on leaves and new growth, soft tissue preferred by sucking insects. The fungus rarely kills plants, but a buildup can prevent light from getting to leaves. The best way to control unsightly sooty mold is to eliminate the insects that secrete honeydew.
The sooty mold that most often affects houseplants is Fumago vagens. Capnodium spp. affects gardenias and azalias. The dark color of sooty mold comes from melanoid pigments in the cell walls of branching, threadlike growth of the fungi.
The four main kinds of insects that suck sap and excrete honeydew on which sooty mold forms are aphids, mealybugs, scale insects and whiteflies.
Aphids have soft, pear-shaped bodies; they are usually green but may be black, brown, pink or yellow. They are about 1/16 to 1/8 of an inch long and feed on new growth and on the bottom of leaves.
Mealybugs, related to scales, are 1/8 to ¼ of an inch long. Feeding mealybug nymphs are covered with a wax coating that helps repel pesticides. Adult females cover themselves and their eggs with a white, waxy material that looks like cotton. Mealybugs feed on the bottoms of leaves and the place where the leaf joins the stem.
Scale insects are from 1/16 to ½ wide and look like fish scales or colored, waxy masses. Immature scales, called crawlers, are able to move. The adults have no visible legs and are immobile.
Whiteflies,1/10 to 1/16 of an inch long, look like powdery white moths. Immature whiteflies look like scales and are immobile; the adults hold their wings at an angle to their body, like tiny roofs.
The tops and bottoms of leaves should be checked for sap-sucking insects when the the plants are watered. A magnifying glass is useful in looking for insects and their eggs and skins they may have shed. Honeydew, a shiny, sticky substance, is usually found on the top of leaves and under the plant.
Stems and large leaves may be cleaned with a soft, moist cloth. Plants with smooth leaves may be washed to prevent the accumulation of dust, which can harbor insects. Plants may be sprayed with lukewarm water, or foil can be used to hold the soil in while the plant is swished in a tub of water containing a few drops of mild dishwashing soap. Using a feather duster may transfer tiny insects and their eggs from one plant to another.
A light infestation of insects may be dislodged by spraying lukewarm water on them. If small numbers of aphids, mealybugs, mites or scales are on smooth leaves, they may be removed with a soft cotton cloth dipped in a solution of ½ tsp. of mild detergent in 1 quart of lukewarm water. Sticky traps may be used to detect the presence of winged aphids and whiteflies.
Horticulturalists at the University of Minnesota recommend insecticides containing the active ingredients of bifenthrin, permethrin or resmethrin for sap-sucking insects. Insecticides containing pyrethrin kill only on contact. Insecticidal soap containing potassium fatty acid also kills only on contact. Neem oil is a botanical insecticide that meets standards for organic growing. Two to three sprayings of horticultural oil every five days may be used to smother the insects.