IDENTIFICATION — Have you noticed a coating that looks like chimney soot on your plants, trees or even the deck or sidewalk? More than likely, it’s sooty mold. This harmless fungus grows on honeydew, a clear, sticky substance secreted by sucking insects, such as aphids, scale, mealy bugs and whiteflies. When these insects attack plants, the honeydew appears on the leaves and sometimes also on nearby surfaces, as well. The sooty mold doesn’t penetrate the leaf, bark or any other surface.
DAMAGE — While most of the damage caused by sooty mold is purely cosmetic, a thick layer on leaves or evergreen needles can reduce a plant’s ability to perform photosynthesis, or feed itself. This weakens the plant, making it more susceptible to other diseases or insects. It’s a particular problem for shade or understory plants, such as rhododendrons, hostas or hydrangeas, which already grow in low light.
CONTROL — To keep sooty mold from coming back, you’ll need to eliminate the insects producing the honeydew. Take a good look above the area where you see sooty mold. The most common sap-sucking culprits you’ll find are aphids and scale. Aphids are easily dislodged with a good strong spray from your hose. Use horticultural oil on scale insects to suffocate them. Some oils should be applied in late winter and others in summer. Also, some plants are sensitive to these oils, so be sure to check the label before you spray.
Once the insects and the honeydew they produce are gone, the mold will gradually wear away from leaf and stem surfaces. To remove stubborn spots on hardscaping, a little scrubbing with soapy water does the trick.