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Is That a “Fungus” on My Tree Trunk?

If you take a close look at some of the more established trees in your landscape, you may notice something you cannot seem to describe – it may look to you like a kind of fungus on your tree trunk. Is it a fungus, and is it harmful to the tree?

The subject of our discussion today may cover some, or a majority, of the tree. It may appear grayish to light green, circular in nature and rather spongy. It sometimes looks like the splat from those spit wads the ornery kids in school used to blow through a straw. And, it’s not moss, which may be more on the north-facing side of the tree.

Some may already have guessed that those are lichens (pronounced “likens”), and they are not harmful to the tree. Still – what are they, and what are they doing there?

According to the Purdue Plant and Pest Diagnostic Laboratory (PPDL), a lichen is a fungus and an alga growing as an interwoven mass that appears to be a single individual. Lichens grow symbiotically, which means that both the alga and the fungus provides something essential to the other for its survival. Lichens are not harmful to plants. They are merely using the plants or trees as a place to anchor.

The U.S. Forest Service states that the alga component of lichens can be either a green alga or a blue-green alga, otherwise known as cyanobacteria. And, they say that many lichens will have both types of algae.

Lindsey Purcell, Purdue Extension urban forestry specialist, recently wrote to offer further insight about lichen components.

“The alga, because it is a green plant, can photosynthesize and provide energy for the lichen,” he said. “The fungus contributes to the relationship by obtaining water and minerals and by protecting the algal cells from desiccation.”

PPDL stated that lichens occur in a variety of habitats from the Arctic to the Antarctic and all regions in between. One finds them on exposed rocks in the deserts, on solidified lava flows in Hawaii, on frozen substrata in the polar regions, on the bark of trees, and on the leaves of plants.

According to PPDL, the most important role of lichens, so far as humans are concerned, is probably as indicators of air pollution. In centers of heavy industrial pollution, no lichens can be found. The lichen population increases gradually with distance from these centers and is thus something of a measure of pollution intensity. So, if lichens are growing in your area, take a deep breath and consider yourself lucky!

PPDL stated that about 18,000 species of lichens have been described worldwide. The U.S. Forest Service estimates that about 3600 species are in North America.

PPDL described three types of lichens:

  1. Crustose(flat, appressed) lichens grow closely appressed to the substrate (i.e. rock or tree trunk) or even within its surface.
  2. Foliose(leaflike) lichens are flattened like leaves but may not be connected to the substrate at all points.
  3. Fruticose(shrublike) lichens have an erect shrublike or filamentous morphology and can be about 10 cm high.

Find information on lichens and other organisms at the Purdue Plant and Pest Diagnostic Laboratory website, Find Purcell’s recent article on lichens at the Purdue Landscape Report website: Find lichen information from the U.S. Forest Service at


Above banner photo: Lichens (center) and moss on tree trunk. Photo by: John E. Woodmansee

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