(News article for August 24, 2024; edited)

Sooty mold grows on many types of plants. Ones on which we often see it include crape myrtles, gardenias, and citrus trees. The fungi that cause sooty mold live on surfaces where certain types of insects have produced a substance called honeydew. Honeydew contains sugars that the insects excrete after feeding on sap in the plant’s phloem tissues.

Insect that produce honeydew include aphids, whiteflies, soft scales, felt scales (e.g., crapemyrtle bark scale), and mealybugs. (Armored scales like the tea scale that gets on camellias do not produce honeydew.) When sooty mold is found on crape myrtle leaves, aphids are often responsible. When it’s found on crape myrtle trunks, crape myrtle bark scale is likely to be the problem. (I address scale insects, specifically, in this article.) On gardenias and citrus trees, whiteflies are often the culprits.

People often object to sooty mold from an aesthetic standpoint. It can also block light from reaching plants, thus reducing photosynthesis, by which plants feed themselves.

If you want to reduce the amount of sooty mold on your plants in the long-term, the insect issue needs to be addressed. (Be aware that, once insects are killed, sooty mold will still remain on the plant for some time.)

Provide good growing conditions for plants, including adequate nutrients, but avoid overfertilizing. Some insects, including aphids, tend to be more problematic when trees have excessive nitrogen.

Predatory and parasitoid insects often help keep populations of pest insects smaller than they otherwise would be. Try to avoid spraying broad-spectrum insecticides that kill these beneficial insects.

Aphids and whiteflies are soft-bodied insects that are susceptible to insecticides like insecticidal soaps and horticultural oils. The same is true for soft and felt scales, but in their adult stages these have some protection from these products due to their waxy coverings. Scale insects are more susceptible in their juvenile stages.

The fact that insecticidal soaps (“potassium salts of fatty acids”) and horticultural oils work only on direct contact with insects is both an advantage and a disadvantage. It’s an advantage from the standpoint of protecting beneficial insects: There is no residual on the plant to harm beneficial insects that come along after the product has dried. However, this lack of residual activity also means that the pest insects won’t be killed if the insecticide doesn’t contact them directly. Since whiteflies fly, this can be a challenge. When using these products, good coverage (including of undersides of leaves) is important, and more than one application may be needed.

Be sure to follow label instructions when applying insecticidal soaps or horticultural oils to avoid injuring plants. These may include precautions about not applying them to stressed plants or above or below certain temperatures.

An option for crapemyrtle aphids, specifically, is to use one of the acephate insecticides that can be mixed into a slurry and painted onto crape myrtle trunks. Acephate moves systemically within the plant, to an extent. Not all acephate products are labeled for use in this manner, so be sure to check the label of any product you plan to buy.

The chemical imidacloprid has efficacy against the mentioned honeydew-producing insects. It’s the active ingredient in several systemic insecticide products sold for application to the soil around trees and shrubs. Imidacloprid moves from roots to stems and leaves. Clearly, being able to apply an insecticide around the base of a plant and have it taken up into the plant is an advantage when trying to manage insects on large trees and shrubs. Using an insecticide that is applied to the soil and taken up by the roots also has benefits from a beneficial insect standpoint. Pest insects come in contact with the insecticide when they feed on the plant, while exposure of beneficial insects that don’t feed on the plant itself is minimized.

However, there are concerns about bees being exposed to imidacloprid in pollen and nectar. Because of this, some imidacloprid product labels require that the product not be applied before or during flowering. Waiting until immediately after a plant has finished blooming for the year maximizes the amount of time between application and flowering. On crape myrtle, this is a limitation from a pest management standpoint, since they don’t bloom until summer, and it takes some time for imidacloprid to be taken up into plants.

Be sure any insecticide you plan to use is labeled for the type of plant on which you plan to use it, and read and follow label directions.

Let me know if you have questions.

Contact Mary Helen Ferguson.

Crape myrtle leaves with aphids, shed aphid skins, and sooty mold. (Photo by M.H. Ferguson)

Crape myrtle leaves with aphids, shed aphid skins, and a lady beetle larva. Predatory insects such as lady beetles and green lacewings often help keep aphid populations smaller than they otherwise would be. (Photo by M.H. Ferguson)