We often get calls at the Extension office from homeowners who ask, “What is this black stuff all over my trees & shrubs?” The answer is sooty mold.
Sooty mold is a problem caused by an infestation of aphids, whiteflies or scale insects. It develops on the foliage of many ornamental plants, covering the top surface of leaves, which decreases the plants photosynthetic ability.
Sooty mold often goes unnoticed until a large number of leaves are covered with an unsightly black goo. It is not really a disease, but a black fungus coating the leaf surface.
Sooty mold is indicative of an insect problem on the plant. The insects feeding on the plant excrete a sugary substance called honeydew. In the summer, when aphid or whitefly populations are high, plants may look shiny and feel sticky. That shiny stuff is honeydew.
Summer rains help rinse this substance away and may even delay the onset of the sooty mold. However, if there is insufficient rain to rinse off the honeydew, it will stick to the leaves. Sooty mold does not feed on plant tissue, but on the honeydew secreted by the insects feeding on the plant.
Aphids, scales, whiteflies and other sucking insects primarily cause sooty mold to form on plants like fig, crape myrtle, azaleas, tulip tree, oleander, and other ornamentals. The feeding of a large number of these insects and the coating of the sooty mold may lead to reduced vigor in the plant. These sucking pests take in large amounts of sap. Much of the water and sugars in the sap pass though the insect and onto the leaf.
To control sooty mold, control the aphids, scales or other pests that are creating the honeydew. A strong spray of water from a water hose can often wash off aphids. This may also remove some of the honeydew and sooty mold. Remaining sooty mold will eventually dry up and flake off the leaves.
To control heavy infestations of aphids, scales and mealy bugs, on ornamental plants, use acephate, imidacloprid, malathion or other recommended insecticides. Read and follow the pesticide label carefully.
Controlling insects will in turn prevent the sooty mold from forming on plants in your landscape. Plan now to take action to prevent this issue from being a problem later in the season.
If you have any questions contact Brenda Jackson at Murray County Extension at 706-695-3031 or email bljack@uga.edu.