Featured Creature: Humongous Fungus

Do you know which fungus among us is most humongous?

Photo from Bib Alex

A Very Large Mushroom

Eleven miles east of Prairie City Oregon, just west of Clear Creek, beneath the ground on a high ridgeline above Malheur National Forest, lives a monster. For thousands of years, this monster has laid beneath the soil, its black tendrils creeping out, searching for victims to infect and consume. Covering almost 3.7 square miles, and possibly weighing as much 35,000 tons, this monster is the biggest living organism, by biomass, on Earth – that we know of, anyway. It is the current “Humongous Fungus” titleholder.

Not that it is the first.

In 1992, a biologist named James Anderson realized that a single 1,500-year-old specimen of Armillaria bulbosa in a forest in Northern Michigan accounted for all the mushroom activity over 37 acres. The realization that one mushroom could grow so large prompted a bit of a race, with candidates for the “Humongous Fungus” title being discovered and tested around the world. In 2003, a group of scientists led by Catherine Parks of the U.S. Forest Service published a paper to announce that a particular example of Armillaria ostoyae in Oregon (one of five candidates in Malheur National Park) covered 2,385 acres, weighed as much as 155 Statue of Libertys, and could be as much as twice as old as the Great Pyramid of Giza. For the moment, it looks like the winner.

Photo from the Oregon Encyclopedia

One Hungry Fungus

While enormous, the humongous fungus can be difficult to find since most of its mass is underground. It grows long, black filaments of fungal tissue beneath the soil called rhizomorphs that spread until they encounter a vulnerable species of tree. The tree will be encircled, and the fungus will squeeze under the tree’s bark, creating paint-like mycelial felts that cut off the flow of nutrients. Eventually, usually 20 to 50 years later, the tree dies, after which Armillaria ostoyae will continue to feed on the dead tree as a saprophyte – a mushroom that feeds on decaying matter. Armillaria ostoyae is a parasite which kills very slowly, and feeds long after its victim is dead.

The simplest way to observe the fungus is to examine its effects on the environment. Large patches of dead and dying trees with the bark coming off, dead branches, crown symptoms such as off-color needles or rounded tops, or excessive coning are all symptoms of Armillaria ostoyae infestation. Searching for distinct groups of dead, dying, or symptomatic trees is the easiest way to appreciate its extent.

In addition, you can look for resin oozing from tree bark, caused by the mycelial felts. Or, in autumn, typically after the first rains, honey mushrooms, named for their color, appear in large clumps coming out of the base of infested trees. These mushrooms are gilled, about 2 to 5 inches high with caps about as wide as the honey mushroom is tall.

Photo by Peter Pearsall from USFWS

How Does It Help?

While not beneficial to trees within their grasp during their parasitic stages, fungi like Armillaria ostoyae play an important role by transporting nutrients through the soil and keeping it moist for young trees to flourish. They remove weak trees and allow resistant genes to flourish, creating a stronger pool of tree genetics. The dying trees also provide food for bark beetles. Ultimately, Armillaria ostoyae plays an important role in clearing out overgrowth and debris.

We Created this Monster

It is important to note that while Armillaria ostoyae may play a beneficial role in the ecosystem, its influence and size appear to be greatly exaggerated by human activity. The natural fire cycle of the forest, nurtured by Indigenous communities, cleared out brush and assisted species such as the lodgepole pine that require extreme heat to sprout. In a healthy forest, the fungus assisted in this regard by removing some of the dangerous tinder.

Logging left massive amounts of debris, though, and 20th century fire suppression allowed that debris to accumulate. Additionally, the replanting of trees using single species and arranging them in a grid-like pattern made it easy for Armillaria to spread. So, while the Humongous Fungus predates 20th century forest management by thousands of years, the natural barriers to its growth were dramatically reduced. As a result, it continues to grow, expanding 5 feet per year in all directions – feeding.

With a more holistic approach to forest management, and a focus on biodiversity, Armillaria ostoyae can return to its rightful place, as a small but crucial part of the forest ecosystem.

Pondering how big the next humongous fungus could be,


Mike Conway is a part-time freelance writer who lives with his wife, kids, and dog Smudge (pictured) in Northern Virginia. 

What the World’s Largest Organism Reveals About Fires and Forest Health – Atlas Obscura
Oregon Humongous Fungus Sets Record As Largest Single Living Organism On Earth – OPB
Humongous Fungus (oregonencyclopedia.org)
Malheur National Forest – Wikipedia
Strange but True: The Largest Organism on Earth Is a Fungus – Scientific American