The Largest Fungus in the World

Written by Colby Maxwell
Published: February 7, 2023
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Fungus is one the least understood members of the living world, even though they play such a vital role in the ecosystem. Without fungus, we wouldn’t have wine or bread, wood wouldn’t break down after it died, and we wouldn’t have anything to put on our salads! Today, we will look at not just the largest fungus in the world but also the largest organism by biomass.

The Largest Fungus (and Organism) in the World

The world’s largest fungus is a honey fungus species called Armillaria solidipes (formerly Armillaria ostoyae). It is also the largest organism by biomass in the world. This fungus is found in the Malheur National Forest in Oregon, covering an estimated 2,200 acres (8.9 square kilometers) of land. It is estimated to weigh around 7,567-35,000 tons and is thought to be between 2,400-8,650 years old. Its name is the Humongous Fungus, which is rather apt for such a massive bit of biology.

The fungus spreads underground by sending out rhizomorphs, thick, black, cord-like structures resembling roots. These rhizomorphs spread out from the original fungus and infect the roots of nearby trees, eventually killing them and growing larger. The fungus is parasitic, meaning it feeds off the host tree, but it also plays an essential role in the ecosystem by breaking down dead wood and returning nutrients to the soil. This massive system of webbing fungus isn’t a single mushroom cap but a network of invisible rot within trees in a massive forest.

How Armillaria Works

Armillaria solidipes is a parasitic fungus that infects and kills trees. The fungus spreads underground through its rhizomorphs, which are string-like structures that resemble roots, although it’s important to remember that fungi aren’t plants but separate organisms entirely. These rhizomorphs spread out from the original fungus and infect the roots of nearby trees. The fungus then colonizes the tree’s root system, penetrating the roots and eventually killing the tree, although it can take some time.

The fungus targets a wide range of trees, including Douglas fir, true firs, Ponderosa pine, and oak. Once the tree is infected, the fungus begins to colonize the root system, penetrating the roots and eventually killing the tree. The fungus also produces mushroom caps, which can be seen growing at the base of the infected tree.

The fungus also plays a vital role in the ecosystem by breaking down dead wood and returning nutrients to the soil. The fungus feeds off the tree and is one of the few living things that can effectively process and break down wood into nutrients.

Armillaria solidipes also has a bioluminescent feature, which means that it produces light, this feature is not well understood, but it is believed to be used for communication and to attract insects for spores dispersal. The mushroom caps that sprout up are the primary organs that use this feature.

Can You Eat Armillaria?

armillaria ostoyae solidipes mushroom cluster
Armillaria ostoyae

is also known as the honey mushroom.

©bogdan ionescu/

Not all species of Armillaria are edible, although Armillaria solidipes is! The mycelium and rhizomes produced by the fungus are found underground and aren’t eaten, but during certain seasons, the fungus will flower and grow mushroom caps above the dirt. These caps are essentially the reproductive organs of the fungus and the part that humans can eat.

The caps are defined as having gills, a campanulate or convex shape, and a decurrent hymenium. Regarding how to prepare these mushrooms, they are best cooked to be safe, although no mushrooms should be eaten without absolute certainty on behalf of the harvester.

How Does Armillaria Reproduce?

Armillaria ostoyae, also known as the honey mushroom, reproduces and fruits through a process known as mycelial growth. The mushroom’s mycelium, or the vegetative part of the fungus, spreads underground through the roots of trees and other woody plants. As the mycelium grows, it forms small, whitish clusters known as rhizomorphs, which resemble roots. These rhizomorphs are responsible for infecting and colonizing new host plants.

Once the mycelium has colonized a host, it begins to fruit, producing clusters of mushrooms found at the base of the host tree or on the ground nearby. The mushrooms typically appear in late summer to early fall and can continue to fruit for several years.

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The photo featured at the top of this post is © LianeM/

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About the Author

Colby is a writer at A-Z Animals primarily covering outdoors, unique animal stories, and science news. Colby has been writing about science news and animals for five years and holds a bachelor's degree from SEU. A resident of NYC, you can find him camping, exploring, and telling everyone about what birds he saw at his local birdfeeder.

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