The 2 384-Acre Mushroom

Imagine a living, functioning organism four-and-a-half times the size of Monaco – one so large that you wouldn’t even know you were standing on top of it unless someone told you.  Beneath a mountain forest in the western United States lies such an organism; one that may just be the largest living thing discovered by man yet.

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Underneath this portion of Malheur National Forest in east Oregon’s Strawberry Mountains lies a giant Armillaria solidipes (also known as Armillaria ostoyae, Armillaria is a very common fungus that grows in temperate North American forests and is a major source of white rot in trees); not that you’d know it.  Walking the ground, an average person may notice a lot of honey mushrooms popping out of the each autumn wherever there are areas of dead or dying trees, but that’s about it.  These mushrooms, however, are merely the fruiting bodies of a giant Armillaria fungus that lies almost entirely underground parasitically feeding on tree roots and conducting nutrients throughout its body over long distances via its own network of ‘roots’ called rhizomorphs; its mushrooms emerging once per year simply to spread its spores throughout the forest.


Honey mushrooms, the fruiting bodies of Armillaria solidipes. Source: W.J. Pilsak,  Licensed under the Creative Commons Attribution-Share Alike 3.0 Unported licence.

First discovered by researchers in 1998, the fungus is spread over 161 km2 (39 784 acres or 62.2 sq mi), with the fungus occupying about 965 ha (2 384 acres or 3.73 sq mi).  Researchers were able to determine that the honey mushrooms popping out of the ground were from the same genetic source by collecting samples of the fungus and growing the samples together in laboratory Petri dishes; the samples reacted to each other normally rather than as alien individuals.  The only way this would not count as the world’s largest organism would be if below the exposed surface the fungus has been broken apart at some point, which would instead make the giant mass a clonal colony (akin to identical siblings in humans).  In any event, based upon the rate of spreading, this particular fungus is anywhere between 1 950 and 8 650 years old.

As Oregon east of the Cascade Mountains is rather dry compared to the west coast, it’s harder for forests to rebound, which means the damage that these huge specimens of  Armillaria solidipes cause to economically-valuable timber resources in the area can be expensive.  As well, the fungus can also feed on dead wood and tree stumps for up to half-a-century.  While fungi are simply part of the natural renewal/decline cycle in forests, the response from the forest industry has been to restock logged stands of fir (the tree most susceptible to white rot due to Armillaria) with more resistant species of wood such as larch and pine.

Researchers had known of such massive fungi since at least 1992, when a 15 ha (38 acre) Armillaria gallica fungusdubbed the ‘Humungus Fungus(sic) was discovered in Michigan’s Upper Peninsula (the Upper Peninsula town of Crystal Falls even holds a Humungus Fungus Festeach August to celebrate the giant mushroom, and the fungus is famous enough to be displayed on U-Haul moving trucks across North America).  Once the Michigan fungus was discovered, other, even larger, specimens were soon found, leading to the belief that such massive organisms are actually the rule for species of Armillaria and not the exception.

Further Reading

Beale, B. (2003).  Humungous fungus: world’s largest organism?  ABC Science, 10 April 2003.  Available at  Accessed 18 April 2012.

Casselman, A. (2007).  Strange but True: The Largest Organism on Earth Is a Fungus.  Scientific American, 4 October 2007.  Available at  Accessed 18 April 2012.

Ferguson, B.A. et al (2003).  Coarse-scale population structure of pathogenic Armillaria species in a mixed-conifer forest in the Blue Mountains of northeast Oregon.  Canadian Journal of Forest Research33(4): 612-603.  Abstract at  Accessed 17 April 2012.

Volk, T.J. (2002).  The Humongous Fungus–Ten Years Later.  Inoculum 53(2): 4-8.  Available at  Accessed 18 April 2012.

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