Definition

A lichen is a non-parasitic organism comprised of a fungus and an algae living symbiotically.

Etymology. 1601, from L. lichen, from Gk. leichen, originally "what eats around itself," probably from leichein "to lick". Originally used of liverwort; the modern sense first recorded 1715.

Discussion

Lichens are often found growing on the surface of rocks or on the trunks of trees. Lichens are not pathogenic to plants, although they may be associated with stressed or debilitated plants. So, there is some debate over whether lichens are a cause or a result of plant stress. Where lichen growth on plants is abundant enough, some premature rotting of bark can occur as water is collected and retained by the lichens.

Management

Lichens are a cosmetic nuisance in residential settings for the eyesores they create on sidewalks, driveways and plants, and for the potential long-term effects they can have upon the bark of trees to which they attach in large quantities. In some cases they can be removed with sprays of pressurized water, applications of approved algicide and fungicide sprays or drenches, and in the case of concrete or asphalt driveways, scrubbing with diluted bleach solutions.

Host range

Lichen species are relatively non-specific organisms capable of colonizing a relatively wide range of organic and inorganic materials. However, there is probably a positive correlation between the occurrence of certain lichen species on certain plant species within certain environments. Thus, the occurrence of lichens is probably more a function of environment than substrate.