The Humongous Fungus

Beneath the soil in the Malheur National Forest in eastern Oregon, the United States, lurks a very large fungus that has been slowly weaving its way through the roots of trees for centuries to become the single largest living organism known to humans.

The fungus, Armillaria solidipes, remains mostly underground, hidden from sight, but every autumn just after the rains it sends up clusters of small yellow-brown mushrooms from the bases of trees it has infected. These mushrooms, commonly called “honey mushrooms”, are the most visible part of the fungus seen by the casual observer. The bulk of the fungus lies underneath the forest floor—a vast network of black filament-like structures called rhizomorphs, that creep through the soil, feeling out new root systems to colonize. The underground growth can stretch up to several square kilometers. The specimen in Malheur National Forest covers 2,200 acres (8.9 square kilometer), and has been named the “Humongous Fungus”. Another specimen, also named the “Humongous Fungus”, resides in a forest in Michigan’s Upper Peninsula. That one is spread over 37 acres.

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The honey mushroom is the above-ground part of a vast subterranean fungus. Photo credit: Dan Molter/Wikimedia

Michigan’s Humongous Fungus was discovered in the early 1990s by a team of researchers working on an unrelated project studying the biological effects of extremely low frequency radio emissions on wildlife in the Upper Peninsula of Michigan. In one particular area of the forest, Armillaria had infected pine trees that loggers had planted years after they removed a batch of infected oak trees. When the researchers collected samples of the fungus from a widespread area and analyzed the DNA, the specimens they collected turned out to be from a single organism.

Before the Humongous Fungus was discovered, mycologists knew that a fungus could grow to be quite large, but no one expected them to grow as large as 37 acres of land. Less than a decade later, a team of researchers from the US Forest Service, trying investigate the cause of a large number of tree deaths in the Malheur National Forest in east Oregon, discovered another Armillaria colony. This one was spread over a phenomenal area—2,200 acres. The fungus was not only large but also very old with date ranging from 2,400 years to 8,500 years.

Scientists are still not sure of all the factors that allow these funguses to become so large and old. Typically, the Armillaria spreads from one tree to another by means of an extensive subterranean network of rhizomorphs, which are black root-like structures. The rhizomorph attach themselves to the roots of living and sometimes dead plants and sap nutrients out of them. They grow just beneath the soil at the rate of approximately 1 meter a year spreading infection from one tree to another even if the trees are spaced out over a large distance. These rhizomorphs give the Armillaria enough competitive advantage over other fungi species. Over the course of several centuries the fungus can expand over vast areas.

The Armillaria fungus isn’t rare. They can be found in forests around the world in North America, Europe, and Asia. So its very possible that even more humongous funguses will be discovered in the future.

If the Humongous Fungus of the Malheur National Forest is considered a single organism, it is the largest known organism in the world by area, and rivals the aspen grove "Pando" as the single largest organism with the highest living biomass.

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Rhizomorphs of the Armillaria fungus propagating up into the wood of a tree. When fresh these rhizomorphs are cream colored but darken to brown or black as they age. Photo credit: nhgardensolutions.wordpress.com

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