armillaria gallica

This Fungus is Even More Humongous Than We Thought

A millennia-old fungal monstrosity has now been estimated to weigh over 400 tonnes. How did it get so big? And what can it teach us?


Back in the early ’90s, Professor James B. Anderson, a University of Toronto biologist and specialist in fungal genetics, led a study of a very special Armillaria gallica specimen better known as the ‘Humongous Fungus’.

One of the largest organisms on the planet, this fungal monstrosity was discovered in a forest in Michigan’s Upper Peninsula. Now, Anderson and his team have returned to the site for a reevaluation a quarter-century on, and they found their original results grossly underestimated its true scale.

Parasitic mega-organism that lurks beneath the soil

The vast majority of the Humongous Fungus is underground, away from direct view. Its underground network, known as the mycelium, uses branching tendrils called rhizomorphs to search the forest for food sources. Soil and decaying plant matter are common food sources, but so are live trees.

Rhizomorphs attach themselves to tree roots and wait for the host to become vulnerable through pestilence, fire, or some other hazard. The parasitic A. gallica then infects the tree and saps nutrients back to the belly of the beast, which grows ever larger.

The original study placed the Humongous Fungus at 100 tonnes, covering 0.37 square kilometres, and around 1,500 years old.

The new estimates are significantly greater: At 400 tonnes, its weight is four times more than previously thought, and it’s spread out across 0.75 square kilometres – the equivalent of 140 American football fields.

Regarding age, it is known that the fungus grew from a single individual, and because of its immense size, it must be older than first believed. By analyzing the growth rate observed in samples extracted onsite, the team placed their new figure at 2,500 years old.

“The defining feature of an individual is its unique genetic code that defines the rule set for its continued existence,” says co-author Johann Bruhn, who discovered the mighty fungus back in the ’80s.

“In this sense, the cell lineage of the Humongous Fungus dates back to a single sexual mating event roughly 2,500 years ago that defines its way of life.”

Anderson has pointed out that 2,500 is a conservative number, and its true age may be much older. It’s possible it could date back to the end of the last ice age around 11,000 years ago, when Michigan had no trees.

“It may go all the way back to post-glaciation when the forest was re-establishing on that site,” he says.

However, if this were the case, the fungus would have had to have spent some time in stasis as opposed to actively growing in order to be consistent with their data.

A special mechanism for mitigating the effect of genetic mutations?

Naturally, the elephant in the room is the question of just how the Humongous Fungus grew to this size and survived over such a long period of time. The team’s prime hunch was that the Humongous Fungus has properties which make it incredibly genetically stable.

Thanks to advances in genetic research techniques, the team were able to test out this idea. Over three years, Anderson’s team collected 245 samples for assessment in the lab.

The rate of mutation in the fungus was found to be close to “impossibly low”, leading the team to speculate that the fungus has a special mechanism which defends against mutations.

More specifically, they think it may compartmentalize mutations in areas where they can’t cause much damage.

“In Armillaria, this would mean that cells in the rhizomorph tip would retain the old DNA, whereas the subtending cells (committed to local, dead-end development) would receive the new DNA. In this way, the rhizomorph tips perpetuating the lineage would retain fewer mutations than cells committed to local differentiation,” according to the paper.

This remarkable genetic stability is in stark contrast to cancer genomes, which are radically unstable.

In a press release, Anderson commented that these polar opposites could make for an interesting counterpoint: “It could be an interesting point of comparison,” he says.

“Cancer is so unstable, mutates at a high rate and is prone to genomic changes, while A. gallica is a very persistent organism with few mutations.”

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Barry is a journalist, editor, and marketer for several media outlets including HeadStuff, The Media Editor, and Buttonmasher Magazine. He earned his Master of the Arts in Journalism from Dublin City University in 2017 and moved to Toronto to pursue a career in the media. Barry is passionate about communicating and debating culture, science, and politics and their collective global impact.