Lichen on Trees May Be a Sign of Decline, But It’s Not the Lichen’s Fault

Southwest Yard & Garden by Dr. Marisa Thompson

Rust-colored growth at the base of this tree in Rio Arriba County is lichen, and it’s not causing any damage to the tree. Photo credit Sid G.

Question: One of our trees has developed a rust-colored stain at the base. Can you tell what this is and what we need to do about it?

- Sid G., Vallecitos, NM (submitted via NMSU Extension Agriculture Agent for Rio Arriba County, Don Martinez)

Answer: That orange growth is lichen, not a rust pathogen. Lichen do not harm the tree or shrub in any way. They tend to grow only on that outer bark and stay there, not growing deeper into live tissues.

Good news: lichen are known to grow in relatively clean air environments and are a sign of low pollution levels. Bad news: tree lichen MAY proliferate on the bark of trees and shrubs that are already decaying/dying from other causes.

Last October at the Trees of the Chiricahua’s Conference in Portal, Arizona, I learned something new about lichen. You may have heard that lichen are interesting because they’re made up of a symbiotic combo of fungi and algae. That’s pretty cool all by itself, but just over a decade ago research by molecular lichenologists showed that there’s an important third component: bacteria.

Part of the reason lichen growth is a sign of low pollution levels could be that those three components (fungal, algal, and bacterial) make the entire organism extremely sensitive to even the smallest changes in the chemical environment.

In an archived Southwest Yard & Garden column written in May 1998, retired Extension Horticulture Specialist Dr. Curtis Smith explained: “If you will notice, there are lichen growing on rocks as well as the trees. The ones on the trees may be closely attached to the branches, or may be foliose, that is, leafy-looking. The ones on the rocks will usually be just closely attached and not foliose. In climates with more moisture, they will even coat the telephone lines in some areas.

“Lichen (the plural as well as singular spelling) do not take food from the plant on which they are growing. You noted that the trees with the most lichen were looking sick. These lichen are a ‘result’ rather than the ‘cause.’ The lichen need sunlight and do better on the trees which are not healthy, so they are more apparent on the ‘sick’ trees. A sick tree will also ‘leak’ nutrients and sometimes sugars and other things that the lichens can use to grow. This causes increased growth of lichen on ‘sick’ trees.

“As for the use of an herbicide on the trees, it may kill the lichen, but will also kill the trees. The best you can do is be sure that the trees receive extra water in the driest times. If there are many trees in the area, you can thin the weak trees by removing them so that the healthy trees can receive extra sunlight and less competition for soil nutrients and water.

“Lichen are interesting but not harmful. Unfortunately, they prefer situations which make them look guilty of harming trees, when they are not the guilty culprit. They are only innocent bystanders taking advantage of the habitat provided by conditions which have harmed the trees.”

Check out the NMSU Extension Guide H-167 “What About the Lichen on My Tree?” for more information (https://aces.nmsu.edu/pubs/_h/H167/).

For more gardening information, including decades of archived Southwest Yard & Garden columns, visit the NMSU Extension Horticulture page (http://desertblooms.nmsu.edu/), follow us on social media (@NMDesertBlooms), or contact your County Extension office (https://aces.nmsu.edu/county).

Marisa Thompson, PhD, is the Extension Horticulture Specialist for New Mexico State University and is based at the Agricultural Science Center at Los Lunas.

Edible New Mexico Magazine article crosslink -- GROWING YOUR FUTURE FAVORITE TREE