Sooty Mold

Pest Notes: Introduction

Sooty mold is a fungal disease that grows on plants and other surfaces covered by honeydew, a sticky substance created by certain insects. Sooty mold’s name comes from the dark threadlike growth (mycelium) of the fungi resembling a layer of soot. Sooty mold doesn’t infect plants but grows on plant parts and other surfaces where honeydew deposits accumulate.

Identification

Fungi that most commonly cause sooty mold on garden and landscape plants are in the genera Capnodium, Fumago, and Scorias. Less common genera include Antennariella, Aureobasidium, Fumiglobus, and Limacinula. The species of sooty mold-causing fungi present are determined by a combination of the environment, host, and insect species present. Some sooty mold species are specific to particular plants or insects, while other mold species might colonize many types of surfaces and use honeydew produced by several kinds of insects.

Honeydew is a sweet, sticky liquid that plant-sucking insects excrete while they feed on sap from a plant. Because the insect can’t completely digest all the nutrients they take in from this large volume of fluid, it assimilates what it needs and excretes the rest as waste (honeydew). Wherever honeydew lands—leaves, twigs, fruit, yard furniture, concrete, sidewalks, parking lots, cars—sooty mold can grow.

A number of insects can produce the honeydew sooty mold needs for growth. These insects include aphids, leafhoppers, mealybugs, psyllids (including eucalyptus redgum lerp psyllid), soft scales, and whiteflies (Table 1). Both the immature and adult stages of these insects produce honeydew while feeding. For detailed information on managing these pests see the Pest Notes listed in References.

Damage

Can sooty mold damage plants or other surfaces? Although sooty mold doesn’t infect plants, the mold can indirectly damage the plant by coating the leaves to the point that sunlight can’t reach the leaf surface. Without adequate sunlight, the plant’s ability to carry on photosynthesis is reduced, which can stunt plant growth. Coated leaves also might prematurely age and die, causing premature leaf drop.

Fruits or vegetables that become covered with sooty mold are still edible. Wash off the mold with mild soap and warm water.

When sooty mold is present on any plants or surfaces in the landscape, it indicates there is, or has been, a sucking insect population present in the vicinity. Most plants will tolerate a small insect population and light amounts of sooty mold. Control of sooty mold begins with managing the insect creating the honeydew.

New, tender plant growth is preferred by many sucking insects and may attract them. Check new succulent plant growth for sucking insects like aphids, psyllids, or any of the other insects listed above. In some situations, a strong stream of water can knock off the insects. Fertilize and water properly to keep plants healthy but not growing excessively.

Another important consideration is ant management. Ants are attracted to and use honeydew as a source of food. They will protect honeydew-producing insects from predators and parasites in order to harvest (collect) the honeydew. Once ants have been eliminated, if predators and parasites are sufficiently abundant, they will quickly begin feeding on and reducing populations of scale insects, aphids, psyllids, whiteflies, or mealybugs.

Careful pruning can be helpful in removing most of the infested plant parts. Ants can be kept out of trees and away from honeydew-producing insects by applying a sticky compound around the trunk and trimming limbs touching buildings or other access points. Baits, such as ant stakes placed under trees and shrubs, may help reduce ant foraging in some cases. More information on ant management can be found on UC IPM’s general Ants web page. Watch this video to see how to keep ants out of trees.

Once honeydew-producing insect numbers are reduced, honeydew production will lessen, and sooty mold will gradually decrease. It can be difficult to eliminate the fungal disease, but sooty mold can be washed off plants and surfaces with a strong stream of water or soap and water.

Table 1. Some common honeydew-producing insects.

Green peach aphids, Myzus persicae	Eucalyptus redgum lerp psyllids, Glycaspis brimblecombe	Adult rose leafhopper, Edwardsiana rosae
Soft brown scale, Coccus hesperidum	Obscure mealybug, Pseudococcus viburni	Sweetpotato whitefly, Bemisia tabaci

References

Dreistadt SH. 2014. Pest Notes: Lace Bugs. UC ANR Publication 7428. Oakland, CA.

Dreistadt SH, Clark JK, Martin TL, Flint ML. 2016. Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide. 3rd Ed. UCANR Publication 3359. Oakland, CA.

Flint ML. 2013. Pest Notes: Aphids. UC ANR Publication 7404. Oakland, CA.

Flint ML. 2016. Pest Notes: Mealybugs. UC ANR Publication 74174. Oakland, CA.

Flint ML. 2015. Pest Notes: Whiteflies. UC ANR Publication 7401. Oakland, CA.

Grafton-Cardwell EE. 2012. Pest Notes: Cottony Cushion Scale. UC ANR Publication 7410. Oakland, CA.

Kabashima JD, Dreistadt SH. 2014. Pest Notes: Scales. UC ANR Publication 7408. Oakland, CA.

Kabashima JD, Paine TD, Daane KM, Dreistadt SH. 2014. Pest Notes: Psyllids. UC ANR Publication 74123. Oakland, CA.

Lawson AB, Dreistadt SH. 2014. Pest Notes: Hackberry Woolly Aphid. UC ANR Publication 74111. Oakland, CA.

Rust MK, Choe D-H. 2012. Pest Notes: Ants. UC ANR Publication 7411. Oakland, CA.

Paine TD, Dreistadt SH, Garrison RW, Gill R. 2006. Pest Notes: Eucalyptus Redgum Lerp Psyllid. UC ANR Publication 7460. Oakland, CA.

Revised based on a previous version authored by FF Laemmlen, UC Cooperative Extension (emeritus), San Luis Obispo Co.