JoAnne Skelly: Sooty mold fungus and you |

JoAnne Skelly: Sooty mold fungus and you

JoAnne Skelly

Since I stop in at the Greenhouse Project every week, I often see plant problems that are out of season for outdoor plants. Recently, I found sooty mold fungus occurring on peppers.

According to the University of California Integrated Pest Management program, “Sooty mold is the common name applied to several species of fungi that grow on honeydew secretions on plant parts and other surfaces…[this] 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.”

Certain insects, such as aphids, leafhoppers and scales, suck out sap from leaves ingesting more than they can assimilate. Then, they excrete the excess. This excess is called honeydew because it is sweet and sticky.

Honeydew is not only deposited on plant surfaces, it also drips all over anything under the plant. In the case of an ash tree infested with aphids in spring, that sticky goo drips all over cars, patios, furniture and anything else underneath it.

Unfortunately, sooty molds can become established on non-living surfaces as well as living ones. Have you ever seen an old wood table or an old statue located under a tree covered with a blackish mold?

While the mold doesn’t infect plants, it can interfere with sunlight absorption and reduce photosynthesis. This could lead to stunted plant growth. Leaves might drop prematurely. Fruits and vegetables are unsightly but still edible. The mold is easily washed off with soap and water.

Other insects we sometime see on house plants that also can cause sooty mold fungus included mealybugs and whiteflies. In the greenhouse, there are aphids, leafhoppers and occasionally whiteflies. What Cory, the Greenhouse Project manager, has been doing to reduce these insect infestations has been to use beneficial insects, either parasites or predators, hosing the plants off and applying an occasional organic insecticide. Other options for the control of sap-sucking insect pests outdoors are horticulture oils, Neem oil or insecticidal soaps.

Targeting pests in an indoor environment requires the use of materials specifically designed for indoor use. Read and follow labels. If you only have a few indoor plants with sticky goo on them, wash them off regularly, on both sides of the leaves. Hand squish aphids and other pests. Sticky traps can help with whiteflies. Dotting scale with isopropyl alcohol on a cotton swab is a doable, but tedious scale control option. Be aware that alcohol may burn some sensitive plants.

For information on sooty mold:

JoAnne Skelly is associate professor and Extension educator, Emerita at University of Nevada Cooperative Extension. She can be reached at