A common question that the Smith County Master Gardener Help Desk often gets asked is, “Why are the leaves (or trunks) black on my plants?” But this year there were new questions about white, fuzzy little “snowflake” insects blowing in the wind, and particularly on and around black-leafed hackberry trees. The little insects were woolly Asian hackberry aphids and the black on the foliage was “sooty mold.”
Sooty mold is a name commonly given to a condition that is not truly a disease, but a black coating on leaves, branches, and fruit made up of fungal growth. The fungus is usually dark colored and powdery-like, hence the name sooty mold. The fungi associated with this condition are saprophytic, that is, they do not feed on live plant tissue, but rather thrive on sticky, shiny insect secretions (poop!) with a high content of sugars. These secretions, known as honeydew, are particularly common with aphids, mealybugs, scale, and whiteflies. The insect honeydew provides nourishment for the fungus and under proper conditions, the entire plant (or plants beneath it) may be covered with the sooty mold.
A black velvety coating made up of the fungal strands is formed on the upper surface of leaves, twigs and fruit. If the honeydew is light, it may appear only in spots. As a general rule, the black fungus coating can usually be rubbed off easily from the surface of leaves, fruit, or branches. With time, the fungus may dry-off, become flaky, and fall off. If for some reason the insect infestation decreases, the amount of sooty mold also will decrease. If no insects are present to cause a re-infestation, rains will usually wash off most of the sooty mold. The fungi causing sooty mold are most common on citrus, crapemyrtles, gardenias, hackberries, and pecans, but can occur on any plant that hosts piercing-sucking insects that secrets honeydew or any plant growing beneath them.
Control can be obtained by applying a properly labeled insecticide that reduces the honeydew producing insect population. Using horticultural or dormant oil formulations as insecticides is often effective, since oil sprays get rid of many of the insect pests and also soften the black fungus so it can be washed off easier by rain or other means. Systemic insecticides (such as Imidacloprid) are also effective but must be properly timed so that the pollinator killing insecticide isn’t inside the plant during bloom time.
It’s important to remember that the black colored sooty mold is not the problem and is not harming the plant. It’s trying to help you by cleaning up the “mess” left by the insects. Honeydew pooping insects are what cause the problem. Once the insects are gone, the sooty mold will gradually fade away as well.