Guilt by association: This often is the plight of lichens in the landscape. Lichens are crusty growths that may be found on just about any outdoor surface, from tombstones and rocks to the bark of woody plants. Lichens are often unnoticed or ignored until a tree or shrub begins to decline. Lichens are colorful and can be unusual looking, drawing attention to a declining plant and leading people to mistakenly infer that lichens caused the plant’s demise. The truth is that lichens are harmless to plants.
Lichens take many forms, which may cause them to be mistaken for plants or for fungi such as mushrooms and molds. Lichens actually are formed by the marriage of green or blue-green algae and a fungus. The two organisms partner together and live as one, each depending on the other for survival.
The roles of the partnership are complex, but, in general, the fungus provides protection and moisture, allowing algae to survive in drier places than they could on their own.
Like plants, algae make their own food through photosynthesis. Fungi cannot make their own food, so they absorb some of the food made by their algal partner. Thus, the fungus provides shelter and water, and the alga provides food. Together they form a lichen. Because lichens make their own food, they do not form a harmful, parasitic relationship with plants.
While lichens themselves do not harm plants, they may indicate that a plant is under stress. Trees that are declining generally have smaller and fewer leaves than normal, allowing more sunlight to reach the trunk and branches. Lichens take advantage of the increased light and become more abundant on the bark. For this reason, many people mistakenly believe that lichens are killing the plant. Actually, lichens are just using the bark as a place to sit and take advantage of the sunlight that they use to make their own food.
Lichens can act as a “canary in a coal mine.” Heavy lichen growth on a tree or shrub may indicate that the plant is under stress from things like disease, insects, environmental conditions, or cultural problems.
Chemical control of lichens on plants is not typically recommended. However, if lichen growth is undesirable, then products containing potassium salts of fatty acids may be applied according to label directions. Examples of labeled products registered for use in Mississippi include Scotts 3-in-1 Moss Control, Bonide MossMax Ready to Spray, and Bayer Advanced 2-in-1 Moss and Algae Killer Ready to Spray. These products control lichens on bark as well as hard surfaces in the landscape, like sidewalks or structures.
The information given here is for educational purposes only. References to commercial products, trade names, or suppliers are made with the understanding that no endorsement is implied and that no discrimination against other products or suppliers is intended.
Publication 2946 (POD-03-19)
By Clarissa Balbalian, Diagnostic Lab Manager, Biochemistry, Molecular Biology, Entomology and Plant Pathology.
Copyright 2019 by Mississippi State University. All rights reserved. This publication may be copied and distributed without alteration for nonprofit educational purposes provided that credit is given to the Mississippi State University Extension Service.
Produced by Agricultural Communications.
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Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. GARY B. JACKSON, Director
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