Have you noticed black mold growing on your lawn furniture, garden umbrella, or other cloth in the landscape? This is sooty mold, so named for the dark pigments in the cell walls of the hyphae. Sooty molds are made up of a complex of fungi, including Cladosporium, Aureobasidium, and Capnodium. We see sooty mold on plants, as well as on furniture and structures. Usually the sooty molds grow on honeydew excrement from sucking insects, although some plants also exude substances that support growth of sooty molds. In the Midwest, if you see sooty mold on a plant, look closer for the insect that is the underlying problem. Plants that might exude sooty mold attractants on their own include catalpa, hibiscus, and juglans species (such as walnut). You might find sooty molds on these plants without insect counterparts.
The photo shows sooty mold growing on a pine tree. The honeydew supporting the sooty mold growth is from scale insects tht have infested this three. The sooty mold growth actually helped alert the grower to a problem with scale insects, so these molds are not all negative. Now, the grower is taking steps to control the insect that has been causing growth decline in his trees.
Sooty mold is unsightly and can interrupt photosynthesis by blocking light. So although sooty molds probably won’t kill plants, they can add to plant stress. Still, most people just find them unsightly. They are more prevalent in drought because many sucking insect populations tend to be higher in drought. In addition, there is less rainfall, so the honeydew is not washed away or diluted.
How can sooty molds be managed? The obvious method is to determine what insect might be present and control the insect. Without the insect, there is no honey-dew, and the mold has no food source. Hosing down plants to remove or dilute honeydew may also help. For outdoor furniture or structures covered with sooty mold, try cleaning solutions that contain a mix of household detergent, mild bleach, and water. Various formulations are recommended on the Internet. Be careful to try the mix on small areas first, and do not let the solution wash onto live plant material.