University of Maryland Extension

Did you hear the story of a fungus meeting algae, and they took a “Lichen” to each other?

Andrew Ristvey, Extension Specialist, Wye Research and Education Center
Lichen on wooden bench

It’s true. The blue-gray and green growth, often seen on the sides of trees and other places like rocks and fences are mutualistic symbiotic organisms collectively called “lichen” (pronounced “liken”). There is no need to fear these organisms as they very seldom to never cause any detriment to the trees they inhabit. Often living on the north side of trees and other surfaces (avoiding heat and thriving on the more moist locations), lichens are naturally occurring and can be quite attractive. An excellent way to tell direction if you are lost in the woods!

Biologically speaking, lichens are a symbiotic, specifically mutualistic, relationship between a member of sac or club fungi and either green algae, cyanobacterium or sometimes brown algae. The mutualism they share begins with the fungi creating an environment for the algae to live, including moisture retention and a substrate to grow. The algae, in return, produce carbohydrates from photosynthesis which the fungi can consume.

What you see on a tree, rock or fence are most often the vegetative structures called thalli (plural of thallus) and look like a scaly mat. Sometimes the reproductive structures called apothecia are noticeable. Lichens can be flat, but are usually three-dimensional and layered, sometimes having structures looking like balloons or like fingers. One can identify specific lichens based on their growth patterns. Lichens have inhabited every corner of the earth. So long as they have a surface on which to form, time to grow and very, very clean air, they do well. Interestingly, lichens are a bellwether for air pollution. If you see lichens growing on trees, it usually means that your air quality is high. So actually, seeing them is a good thing. Additionally, many animal species utilize lichens from nesting material to food.

Do lichens damage plants? Not typically. But, as noted in an Ohio State Extension Bulletin (Special Circular 195-029), they may have some negative effects. Since they are encrusting life-forms, they may possibly be unsightly on trees that are grown specifically for attractive bark. However, the beauty of lichens on bark should also be considered and appreciated. In some cases lichens may have an indirect and undesirable affect when they may be part of a pest’s life cycle as in the case of the hemlock looper, (Lambdina fiscellaria). Additionally, algal and fungal plant pathogens which cause scurfy leaf spots and fissured twig cankers on many plants may actually form a symbiotic relationship similar to lichens. However, this is not the norm.

Lichen control, if necessary, can be done by management of plant health. Lichens can be an indicator of declining plant health where thinner canopies increase light penetration to the branches, promoting lichen growth. Pruning declining limbs may also be a way to slow the progress of the lichen. Kocide® is a fungicide/algicide that is labeled for use on lichen on specific conifers only, but no other chemicals are labeled.

For what it is worth, teach your clients that lichens are a benevolent part of nature in most cases, and that they show how healthy the environment is around them.

Information from this article was partially retrieved from the Ohio State University Extension Bulletin Special Circular 195 – 029 by David J. Goerig and James A. Chatfield.

October 2013
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