The Story of the World’s Largest Fungi

Armillaria soldipes mushrooms

In a forested area along the western edge of Michigan’s Upper Peninsula lurks a very large fungus. Every year it sends up clusters of small, light-brown mushrooms, but the bulk of the fungus is buried underground, hidden from sight. Just beneath the surface of the forest floor millions of fuzzy, white fibers called hyphae sap nutrients from tree roots, and miles of black, string-like structures called rhizomorphs reach through the soil, feeling out new root systems to colonize. All told, this subterranean fungal network sprawls across 37 acres of land. If it were growing beneath central Boston, it would occupy most of the Boston Common. It doesn’t inspire awe in the way that a giant redwood or blue whale might, it isn’t grand, and it isn’t majestic, but this prodigious fungus is among the largest living things in the world.

Michigan’s Humongous Fungus, as it has come to be known, is no longer the world’s largest known fungus, but it was the first mega-fungus ever discovered. Biologists Myron Smith and James Anderson, both of the University of Toronto at the time, became aware of the fungus’ true size almost by accident over 20 years ago. The story begins in 1988, when they were working on an unrelated project studying the effects of extremely low frequency radio emissions on wildlife in the Upper Peninsula of Michigan. One of their research sites was an active logging area outside a quiet town called Alpha. The old growth forest that once occupied the site had been cut down years earlier, and the pine seedlings that were planted to replace the felled trees had begun to die off. The seedlings’ deaths were surely an unwelcome surprise for the logging company that planted them, but for mycologist Johann Bruhn the deaths were no mystery. Bruhn had spent several years studying various fungi that grow at the site, and he was all too familiar with one of the site’s more plentiful fungi, and the prime suspect in the deaths of the seedlings, a type of Armillaria fungus.

Fungi in the genus Armillaria are commonly known as honey mushrooms. Although the mushrooms are edible, the name has more to do with their color than their taste, which is apparently quite bitter. Most species of honey mushroom grow on tree stumps, fallen branches, and other dead wood, but a few species infect the root systems of living trees as well. And it was one of these more sinister varieties of honey mushroom that was killing seedlings outside Alpha. Before the old-growth forest was cut down, the established trees were infected with a honey mushroom variety known as Armillaria gallica. The fungus leeched nutrients from the trees’ roots, but the trees were large and otherwise healthy enough that they were in no real danger of succumbing to the infection. The fragile seedlings, on the other hand, never really stood a chance.

Finding themselves in the midst of a rather sizable population of fungi, Smith and Anderson seized the opportunity to try to answer some questions they had about how fungi pass mitochondrial DNA on to their offspring. So in collaboration with Bruhn they began collecting samples of Armillaria gallica. The sampling technique was straightforward: they stuck wooden popsicle sticks into the ground, waited a few days for the fungus to colonize the sticks, and then collected them.

During their first season at the site, Smith, Anderson, and Bruhn collected fungal samples within a roughly rectangular area measuring 120 by 60 meters. As they set about analyzing the samples, they slowly began to suspect that nearly all of their samples were taken from one, individual fungus. The samples shared identical mitochondrial DNA, and when they were placed next to one another in petri dishes, they grew together seamlessly, a sign that the samples were genetically identical.

Before the Humongous Fungus was discovered, mycologists knew that a fungus could theoretically grow to be quite large, but no one guessed that one might grow to occupy 37 acres of land. Unlike humans and other mammals, which reach their adult size and stop growing, fungi can continue to grow larger throughout their lives. Armillaria fungi are particularly well-equipped to grow to enormous sizes because they are capable of producing black, root-like structures called rhizomorphs. The fungi use rhizomorphs  to spread through nutrient-poor expanses of soil, seeking out new root systems and dead wood to colonize. This means that Armillaria gallica, unlike many other fungi, can spread from one tree to another even when the trees are not in direct contact. This ability, given enough time and favorable growing conditions, allows the fungus to expand over vast areas.

Armillaria rhizomorphs

By the time Smith, Anderson, and Bruhn returned to the Alpha, Michigan site the following year, they had expanded their research focus from ‘how do fungi inherit mitochondrial DNA in the wild?’ to ‘just how big is this fungus, anyway?’ That year they collected fungal samples along a one kilometer transect cutting through the previous year’s rectangular sampling area. Again, all the samples were genetically identical. As they expanded the sampling area over subsequent research seasons, the outer boundaries of the fungus began to emerge.

The claim that the Humongous Fungus was one of the largest organisms in the world prompted a debate among some scientists about how to define the word ‘organism.’ We’re used to thinking of an organism as something with a defined size and shape, something we can see in its entirety. But the Humongous Fungus doesn’t match up with this view. It’s a nebulous organism, composed of tiny, branched fibers called hyphae, which gather into moldy-looking clumps and mats, or are bundled together into specialized structures like rhizomorphs and sometimes mushrooms. The body of the fungus is called the mycelium, but it is a body of indeterminate size and shape, growing outwards in all directions, interspersed with tree roots, stones, dead leaves and everything else you might find in the soil beneath a forest. It’s difficult to even imagine what the entire fungus looks like, since we can only see small pieces of it at any one time: a cluster of mushrooms here, or a filamentous mat creeping up the base of a tree there.

Some scientists wondered if the Humongous Fungus can be called an individual organism simply because it is genetically homogeneous. In addition to genetic uniformity, there are other criteria that scientists might use to decide whether the Humongous Fungus acts as a single organism: Can different parts of the fungus communicate with one another, despite being up to a kilometer apart? Does the fungus redistribute resources from one part of its mycelium to another where they are needed more urgently? Unfortunately there are currently no clear answers to these questions. Even now the structure of the Humongous Fungus remains a mystery. It may be a single, interconnected whole. Or, as it expands and retreats with the growth and death of the tree roots it feeds upon, parts of the fungus may become isolated from the rest of the mycelium, only to grow together again years later.

It seemed that the most obvious explanation for the genetic similarity among the samples that Smith, Anderson, and Bruhn had collected was that all of the samples were taken from one, giant fungus. But there was another possibility. A new Armillaria gallica individual is born when two spores from two genetically distinct “parent” fungi come into contact. This means that it’s possible for two or more individual fungi to share the same parents, and you can imagine that these “sibling” fungi would have similar genetic profiles. Perhaps the Humongous Fungus was composed of a series of siblings? Anticipating this possibility, Smith, Anderson, and Bruhn performed additional genetic tests. They found that the level of genetic similarity shared among their samples was so great that it was impossible for the samples to have come from two or more sibling fungi. The only remaining conclusion was that the Humongous Fungus grew from a pair of microscopic spores to its current size. Using the measured size of the fungus and an approximation of its average annual growth rate, Smith, Anderson, and Bruhn estimated the age of the fungus to be more than 1,500 years, making it not only one of the largest living things, but also one of the oldest.

Armillaria fungi aren’t rare. They can be found in forests around the world, in North America, in Asia, and in Europe. Smith, Anderson, and Bruhn weren’t so naive as to think that the first fungus they measured happened to be the largest fungus in the world. So they weren’t surprised when an even more humongous fungus was discovered nine years later hidden beneath the Malheur National Forest in eastern Oregon. It was a member of an Armillaria species with the taxonomic name Armillaria soldipes, and like the Michigan Humongous Fungus it drew researchers’ attention because it had killed an impressive number of trees. In fact, it left a swath of arboreal destruction so large that the damage was best viewed from an aircraft. Examining photographs taken at an altitude of several thousand feet, researchers could see a patchwork of ring-shaped holes in the forest canopy. Many of these circular patches of dead trees were separated by considerable distances. Here was visual evidence of the fungus’ ability to stealthily jump from one group of trees to another, far-off grouping without seeming to affect the trees in between. Genetic testing revealed that the Oregon fungus is 65 times larger than the Michigan fungus. It covers 2,400 acres, an area almost three times larger than Manhattan’s Central Park, and its age has been conservatively estimated at 2,400 years.

The life of the Michigan Humongous Fungus began with the chance union of two windblown, microscopic spores. Meanwhile, thousands of miles away, the Roman Empire was falling. As the fungus slowly expanded over the centuries, it survived the death and rebirth of the plants and trees above it an untold number of times. Wildfires swept the landscape clean. Subtle shifts in climate brought changes in the density of the forest and the types of plants and trees that grew there. Logging companies arrived and cut down many of the forest’s largest trees. Scientists plucked tiny bits of the fungus from the ground. But through everything the fungus has persisted. It might very well continue its slow, subterranean creep for another 1,500 years.

References and Further Reading

Volk, T.J. (2002). The Humongous Fungus – Ten Years Later. Inoculum 53(2): 4-8.

Gould, S.J. (1992) A Humongous Fungus Among Us. A thought-provoking essay by Stephen J. Gould about what individuality means and how the discovery of the Humongous Fungus challenged our understanding of the concept.

Smith, M.L, Bruhn, J.N., & Anderson, J.B. (1992). The fungus Armillaria bulbosa is among the largest and oldest living organisms. Nature 356: 428-431. DOI: 10.1038/356428a0

Ferguson, B.A., Dreisbach, T.A., Parks, C.G., Filip, G.M., & Schmitt, C.L. (2003). Coarse-scale population structure of pathogenic Amillaria species in a mixed-conifer forest in the Blue Mountains of northeast Oregon. Canadian Journal of Forestry Research, 33: 612-623. DOI: 10.1139/X03-065  (pdf)

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