How to Manage Pests
Pests in Gardens and Landscapes
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.
Fungi that most commonly cause sooty molds in garden and landscape situations are in the genera Capnodium, Fumago, and Scorias. Less common genera include Antennariella, Aureobasidium, and Limacinula. The species of sooty molds 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 others might colonize many types of surfaces and use honeydew produced by several kinds of insects.
A number of insects can produce the honeydew sooty molds need for growth. Their common characteristic is that they all suck sap from plants. The insects include aphids, leafhoppers, mealybugs, psyllids (including eucalyptus lerp psyllid), soft scales, and whiteflies. Both the immature and adult stages of these insects feed by sucking sap from plants, producing honeydew.
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.
Sometimes judicious pruning can be helpful in removing most of the infested plant parts. Also, keep ants 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 in Pest Notes: Ants.
Once honeydew-producing insects are suppressed, sooty molds will gradually weather away. In some instances, if necessary, sooty molds can be washed off with a strong stream of water or soap and water. However, it can be difficult to remove sooty mold even with soap and water.
Dreistadt, S. H., J. G. Morse, P. A. Phillips, and R. E. Rice. March 2007. Pest Notes: Scales. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7408.
Dreistadt, S. H., and E. J. Perry. Aug. 2006. Pest Notes: Lace Bugs. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7428.
Flint, M. L. May 2000. Pest Notes: Aphids. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7404.
Flint, M. L. Sept. 2002. Pest Notes: Whiteflies. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7401.
Grafton-Cardwell, E. E.. Dec. 2003. Pest Notes: Cottony Cushion Scale. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7410.
Paine, T. D., and S. H. Dreistadt. Aug. 2007. Pest Notes: Psyllids. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7423.
Paine, T. D., S. H. Dreistadt, R. W. Garrison, and R. Gill. Jan. 2006. Pest Notes: Eucalyptus Redgum Lerp Psyllid. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7460.
UC Statewide IPM Program. Feb. 2007. Pest Notes: Ants. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 7411.
Author: F. F. Laemmlen, UC Cooperative Extension (emeritus), San Luis Obispo Co.
Produced by UC Statewide IPM Program, University of California, Davis, CA 95616
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