Sooty mold is the common name applied to several species of fungi that grow on honeydew secretions on plant parts and other surfaces. The fungi’s dark, threadlike growth (mycelium) gives plants or other substrates the appearance of being covered with a layer of soot.
Sooty molds don’t infect plants but grow on surfaces where honeydew deposits accumulate. Honeydew is a sweet, sticky liquid that plant-sucking insects excrete as they ingest large quantities of sap from a plant. Because the insect can’t completely utilize all the nutrients in this large volume of fluid, it assimilates what it needs and excretes the rest as “honeydew.” Wherever honeydew lands—e.g., leaves, twigs, fruit, yard furniture, concrete, sidewalks, or statuary—sooty molds can become established.
Although sooty molds don’t infect plants, they can indirectly damage the plant by coating the leaves to the point that it reduces or inhibits sunlight penetration. Without adequate sunlight, the plant’s ability to carry on photosynthesis is reduced, which can stunt plant growth. Coated leaves also might prematurely age (senesce) and die, causing premature leaf drop.
Fruits or vegetables covered with sooty molds are edible. Simply remove the mold with a solution of mild soap and warm water.
Most plants will tolerate a small insect population and light amounts of sooty mold. When sooty molds are present on any surface in the landscape, it indicates there is, or has been, a sucking insect population present in the vicinity. Control of sooty molds begins with managing the insect creating the honeydew. For example, populations of aphids usually are highest on succulent, new growth. In some situations a strong stream of water can dislodge the insects. Also fertilize and water to keep plants healthy but not excessively vigorous.
Another important consideration can be ant management. Ants are attracted to and use honeydew as a source of food. Because of this, they will protect honeydew-producing insects from predators and parasites in order to harvest the honeydew. In many cases, predators and parasites are sufficiently abundant and quickly begin feeding on and reducing populations of scale insects, aphids, psyllids, whiteflies, or mealybugs once ants have been eliminated. If populations fail to decline, apply horticultural oils, neem oil, or insecticidal soap to suppress the problem insects. One or more applications might be needed. For detailed information on managing these pests see the appropriate Pest Notes listed in References.