Sooty mold is a black substance, coating orange tree leaves and fruit. It looks like mold and wipes off easily. The mold can live on a plant's natural secretions but the likely cause for the mold's presence is honeydew secreted from sucking insects such as aphids, mealybugs, psyllids or scales. Any plant with these insects present is at risk of developing sooty mold.
For small sooty mold infestations, simply wiping the leaves will get rid of the mold and the insects.
Areas that frequently have cool, moist days are at higher risk for sooty mold than other areas. The mold in sooty mold does not cause a plant disease.
The key to control sooty mold is controlling the sucking insects. First give your plant or tree a good hard spraying of water to wash away some of the mold and insects. To treat a large tree, add 4 ounces of dish-washing liquid to a hose end sprayer and spray the tree early in the morning. Always check a plant for burning before using soap spray, but most shiny leaf plants can tolerant soap.
Less than 2 percent of the insects in the world are harmful. Most are beneficial.