Sooty molds can develop on almost any plant. They are caused by fungi that grow in the sticky honeydew left on the foliage by feeding from sap-sucking insects, such as aphids, mealybugs, and scales. Sooty mold is relatively harmless to plants, because the mold derives its nutrition from the honeydew rather than the plant itself. However, the black coating cuts out some light to the leaf, so photosynthesis is reduced. In severe cases, the leaf may yellow from lack of light. More serious is the insect infestation that caused the honeydew. Aphids may be easily controlled with insecticidal soap or dislodged with a forceful water spray. Mealybugs and scales can be more difficult to manage. Use an insecticide labeled for treatment of these pests to control them. Remove sooty mold from plants by wiping leaves with a wet rag or washing it off with a forceful spray of water.