By James A. Bethke, for Let's Talk Plants! March 2022.
I gave a presentation a few years back at a community center, which was surrounded by some beautiful trees. However, I noticed that the sidewalks and the plants below the trees were all black and quite unsightly. The black sooty mold all around the base of the plant was a good indicator of a pest insect nearby, and upon inspection, I observed an extreme infestation of black scale on the tree stems. The stems were nearly completely covered in scale.
Black sooty mold can also be quite common in the landscape and garden, especially if the pest has gone unnoticed. Sooty mold is fungi growing on leaves, stems, and other surfaces covered by honeydew. Honeydew is the excrement of a select group of piercing-sucking insects that feed on the phloem (vascular tissue) that transport nutrients throughout a plant. These insects include aphids, some leafhoppers, mealybugs, some psyllids, soft scales, and whiteflies. They will digest as many nutrients as they can from the phloem, but they excrete most of the sugars, which is why many ant species will tend these insects to exploit the sweet food source. When the insects are in great numbers, honeydew builds below their feeding sites.
The first thing you will see is a shiny, sticky substance (honeydew) on the upper surface of leaves or stems (Figure 1). Wherever honeydew lands (leaves, twigs, fruit, yard furniture, concrete, sidewalks, parking lots, cars) sooty mold can grow.
The second thing you will see is mold growing on the honeydew (Figure 2). The figure is of a poinsettia leaf that is below a whitefly infested leaf. Whiteflies feed on the undersides of leaves and produce lots of honeydew. Fungal spores are tiny and found in incredible numbers in the air we breathe, and they land on the honeydew. Not all fungi will grow on the honeydew, but the common fungi that produce sooty mold are in the genera Capnodium, Fumago, and Scorias.
The third thing that you will see is black sooty mold (Figures 3 & 4). Black sooty mold’s name comes from the dark threadlike growth (mycelium) of the fungi resembling a layer of soot. Sooty mold does not harm plants because its food is the honeydew and not the plant. However, in great quantities sooty mold can block sunlight and chlorophyl food production. It is an aesthetic damage as well because the infested area can be very unsightly.
What can you do to avoid black sooty mold? Controlling the insect pest and the ants that protect them will go a long way in preventing honeydew and the eventual black sooty mold. If honeydew is present, it can be washed off with plenty of water, especially if it is a cool wet morning, but black sooty mold is a late-stage effect that cannot be washed off. Unfortunately, black sooty mold will likely be on the plant until the infested areas are removed from the plant or die off.