Ask The County Agent: Is this mold covering my trees and shrubs?
By Stephanie R. Butcher
County Coordinator Agricultural & Natural Resources Agent
Question: What is this black stuff covering the leaves of my ornamental trees and shrubs?
Agent: You are probably seeing sooty mold. It’s not really a disease. It’s a fungus that is caused by insects feeding on the plants. These sucking insects take in large amounts of sap. Much of the water and sugars in the sap pass through the insect and onto leaves as a sugary substance called honeydew. When populations are high enough, the honeydew covers the leaves and makes them look shiny and feel sticky to the touch.
The fungus feeds on this sticky honeydew that is left by insects and covers the top surface of leaves in a black substance called sooty mold. Because summer rains help rinse away the honeydew, it can delay the onset of the fungus and homeowner’s don’t usually notice it until late summer or early fall when insect populations are high and there are fewer rains to wash away the honeydew and sooty mold.
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 plant vigor by reducing photosynthesis, but it won’t usually kill the plant.
To control sooty mold, you must control the insect 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 insecticides labeled for the insects that are present. Read and follow pesticide labels carefully and contact the Extension office if you need help with insect identification.
Controlling insects will prevent sooty mold from forming on plants in your landscape, so plan now to prevent this issue from being a problem next season.
For more information about sooty mold or caring for ornamental plants in the landscape, contact the Coweta County Extension office at 770-254-2620 or [email protected].
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Feature Photo Credit: University of Georgia Cooperative Extension