What Is Sooty Mold? How to Get Rid of It and Save Your Plants

If you’ve noticed a weird black coating on plants in your garden, you may be dealing with sooty mold. “Sooty mold is a group of fungi that cover the surface of the plant,” says Peng Tian, Ph.D., assistant extension professor and lab director of the Plant Diagnostic Clinic of the University of Missouri. “It’s not just one fungus; it’s a big party of them. It’s superficial and doesn’t actually penetrate the leaves.”

Sooty mold can affect many different trees and shrubs such as dogwoods, camellias, azaleas, gardenias, and crape myrtles; it doesn’t typically affect annuals or vegetables. The fungi can darken the surface of leaves, stems, branches, and trunks. The unsightly black coating may stress the plant by blocking its ability to photosynthesize, or make food, says Tian.

Interestingly, the underlying problem isn’t the fungi. “It’s insects feeding on the plants,” says Rafia Khan, Ph.D., assistant professor and extension entomologist at Texas A&M AgriLife Research and Extension Center. “Pests such as aphids, whiteflies, mealybugs, and soft scales use their sucking mouthparts to remove sap from the plants. Then they excrete honeydew, the excess water and sugar they can’t use from the sap, onto the plants.”

This shiny, sticky honeydew is what provides food for the fungi. Because the honeydew can drip anywhere, you sometimes see sooty mold develop on other surfaces, such as concrete walkways, patio furniture, or even your car, if parked under shrubs or trees, says Khan.

The good news is that sooty mold doesn’t just appear overnight. “The insects start and their numbers grow, slowly building up a layer of honeydew,” says Cory Tanner, extension horticulturist at Clemson University. “At minimum, sooty mold can take months to develop, so you have time to figure out what’s going on.”

Wondering how to know if you have sooty mold? Here’s the scoop—plus, how to get rid of it.

Your first step is to inspect plants closely. Look for insects, which are the source of the honeydew. Examine leaves, stems, and crevices closely. When you find an insect, get a positive identification so you’ll know what management techniques you need to use, says Khan. Your local university cooperative extension service can help identify the culprits.

You also may need to do some deep investigating. Sometimes a plant covered in sooty mold isn’t the one infected. It’s located beneath the plant that’s actually affected by the insects. For example, boxwood shrubs covered in sooty mold may be growing under a crape myrtle that’s infected with scales, says Khan.

To control sooty mold, get rid of the pests that are creating the honeydew first. A blast from the garden hose can often take out aphids, which are tiny soft-bodied bugs. However, you may need to treat with a narrow-spectrum insecticide, such as horticultural oil, insecticidal soap, or neem oil to get rid of other pests. Be sure to follow the label instructions exactly, says Khan.

Once you’ve managed the pest, the sooty mold will dissipate because there’s no longer a food source—though it may take a while for it to flake or fall off. If you would like to remove the fungi after dealing with the pests, spray soapy water (1 teaspoon of soap per 1 gallon of water) on plant surfaces, then rinse off with the hose, says Khan.

It’s unlikely that you’ll be able to totally prevent sooty mold, as the fungi are always present in the environment. But conditions do change from year to year. “It’s also usually not necessary to treat with a fungicide, which is typically better used as a preventive measure,” says Tian. “Also, disease pressures may not be as heavy next season.”

The most important thing to remember is to monitor your trees and shrubs. Walk around your garden and keep an eye out for anything that looks off. “Pay attention to what’s going on. If you notice the sticky honeydew before the sooty mold develops, rinse it off with the hose right away,” says Tanner.

Although sooty mold inhibits the plant’s ability to photosynthesize, it’s not likely to kill the plant. The greater risk often is the insect infestation, which definitely stresses and weakens the plant, says Tanner.

Sooty mold can be especially tough on evergreens because they don’t shed their leaves, so the discoloration may last for some time. “You almost need to wait it out for these plants to recover their green appearance, though a very light pressure wash also may remove the sooty mold if you’ve dealt with the insects,” says Tanner.