This common black mold is found on a wide variety of plants in the garden. It is caused by any of several species of fungi that grow on the sugary material left on plants by aphids, scales, mealybugs, whiteflies, and other insects that suck sap from the plant. The insects are unable to digest fully all the sugar in the sap, and excrete the excess in a fluid called honeydew, which drops onto the leaves and fruit below. The sooty mold fungus develops on the honeydew, causing the leaves to appear black and dirty. Sooty mold is unsightly, but it is fairly harmless because it does not attack the leaf directly. Extremely heavy infestations prevent light from reaching the leaf, so that the leaf produces fewer nutrients and may turn yellow. The presence of sooty mold indicates that the tree is infested with insects.
Sooty mold will eventually be washed off by rain. Prevent more sooty mold by controlling the insects that are producing the honeydew. Inspect the foliage to determine what types of insects are present. For control instructions: for aphids, Aphids; for scales, Soft Scales; for whiteflies, Whiteflies; and for mealybugs, Mealybugs.
