Updated: Sep 24, 2021
By Dr. Judy Stout, Mobile County Master Gardener |
Q Since recent storms have blown many leaves off my shrubs and trees, I have noticed an attack of grey-green stuff growing on the stems and trunks of many plants. I’m afraid it is killing the plants. I am sending pictures of some of it. What is it and how do I get rid of it?
A From your pictures, I can tell the growths you have concerns about are several kinds of Lichens. First, let me assure you that lichens are not a disease and are not parasites that are going to damage your plants nor cause them to die.
Lichens represent, in fact, a very special kind of biological cooperation that creates a single “organism” from two or more distinctly different life forms, representing two Kingdoms, plants and fungi. Each lichen type is composed of a fungus and an alga, usually a green alga, neither of which can exist free-living without the other. Occasionally, a cyanobacteria (from a third Kingdom, bacteria), is also found in the structure. This is called symbiosis and mutually benefits each partner. Threads of the fungus compose most of the structure with the other partners embedded within. Fungi have no chlorophyll and cannot produce their own food, while the algal and cyanobacterial components are fragile if bared to the environment. In partnership, the fungus provides protection, absorbs and retains moisture and nutrients from the air, and anchors the lichen while the algae produce food for the whole structure. Lichens may represent the pinnacle of evolutionary cooperation. There is evidence that they have existed for over 400 million years!
Lichens are using your trees and shrubs only as solid structures to hold onto. They can be found on almost any hard substrate – rocks, fences, birdhouses, dry soils, headstones, walls, etc., not just on living plants. Lichens may appear as fuzzy, hairy or leafy bodies, as scales, or as just a colored patch that cannot be separated from what it is growing on. Red, green, yellow, orange, grey, white, or grey-green lichens may be growing on the same surface or on nearby surfaces. They are common and have been described as “obvious but not always noticed.” They are probably being noticed now on your plants because many leaves are gone and have been hiding the presence of the lichens. In our area, lichens are often observed on pecan trees, azaleas, and dogwoods.
Sometimes, container plants of deciduous (without leaves in the winter) species may seem to be under attack by lichens. Again, this is seasonal awareness of something natural and not necessarily indication of a ‘sick’ plant. Remember this when shopping nurseries or plant sales before deciduous plants have leafed out.
Lichens may be found on plants that appear to be damaged or dying and are often blamed for the poor condition of the tree or shrub. They were probably there anyway and are just taking advantage of increased light made available by a thinning canopy of leaves but not causing the decline in the host plant. These symptoms may be because of poor plant management (e.g., moisture, fertilization, intense heat or too much light), disease, or pest infestation. Improved plant care is the solution. Proper pruning can increase the density of the leaf canopy.
It has been estimated that there are between 13,000 and 18,000 species of lichens, worldwide. They are found on all continents, in all climates, and in the most severe environments, from the Arctic tundra to harsh deserts. Lichens are long-lived (up to 1,000 years or longer) and slow- growing. They require three things – light, clean air, and something to attach to. They can survive long periods of drought and can even extract moisture from fog and dew. They are opportunists, spread by their fragments settling wherever suitable conditions occur.
Lichens have significant cultural, environmental, and economic impacts. They have been widely used for centuries by indigenous communities. Some chemical byproducts from lichens have been used for their medicinal properties (especially as antibiotics) and continue to be investigated for medical uses. Other byproduct chemicals weather rocks and produce soil, cause deterioration of headstones, sculptures, and ancient human structures. Pigments from lichens are useful in dyes and are still valued for the brown and green colors in Harris tweeds. Manufacturing of litmus paper still depends on lichen pigments. Although not often consumed as human food, lichens represent about 90% of the winter diets of caribou and reindeer and may be consumed by mountain goats and flying squirrels. Sensitivity of lichens to air pollutants, such as ozone and sulfur dioxide, provides a reliable indicator of air pollution and can be used to monitor progress in recovery from air pollution.
Lichens: What interesting organisms! So unique and diverse. Fun to look for and see how many kinds can be found and in how many places. NOT TO BE FEARED!
Foliose lichen with its cup-shaped spores