The Secrets of the 'Humongous Fungus'

How one of the biggest living organisms in the world got so big

Mushrooms growing on a tree
Mushrooms from Armillaria mellea growing on a living tree  (Virag Tomity)

Twenty-five years ago, James Anderson discovered a fungus that expanded the possibilities of life on Earth.

It was a single fungus of the genus Armillaria, weighing an estimated 22,000 pounds and spread over a remarkable 15 hectares. The organism had been growing for around 1,500 years, more than a millennium before the land under which it grew even became the state of Michigan. When Anderson and his collaborators wrote it up in Nature, they suggested it was “among the largest and oldest living organisms” in the world.

This suggestion, in its use of superlative, set off a competition—naturally. Scientists all over the world were soon hunting for Armillaria, or honey mushrooms, in their own forests. The title of the largest fungus in the world eventually went to one in the Malheur National Forest in Oregon: nearly 1,000 hectares big, as many as 8,650 years old. This “humongous fungus,” as it’s sometimes called, is by some counts still the largest living organism ever found.

Fungi normally grow as mycelia—soft, white, cottony tufts that you may have seen on food kept in the fridge too long. Some of them also form mushrooms. But Armillaria, somewhat uniquely, can also grow thick, black, rootlike rhizomorphs whose networks can extend miles through the soil in search of wood to eat. The rhizomorphs, scientists think, are what allow a single Armillaria organism to get so big. A comprehensive new genetic study takes up the question of how Armillaria got its rhizomorphs.

“Ever since I since I was a graduate student, I wanted to do exactly the study that was just published,” says Anderson, who is now a professor of biology at the University of Toronto. Anderson contributed a couple of genomes for the study, but the bulk of the research and analysis was done by György Sipos and László Nagy, of the University of Sopron and the Hungarian Academy of Sciences, respectively.

Sipos and Nagy not only sequenced four species of Armillaria, but they also pinpointed genes active in the fungus’s rhizomorphs and mushrooms. To identify those genes, they had to figure out how to grow at least one species of Armillaria in a lab.

Rhizomorphs of Armillaria mellea (Lairich Rig)

The rhizomorphs were the easy part. Once Armillaria took to their growing medium of rice, sawdust, tomato, and orange—“this fungus has really weird tastes,” notes Nagy—they spontaneously formed rhizomorphs. The mushrooms were much trickier. They had to trick the fungus into thinking it was autumn, which they did by moving their fungi to colder temperatures with progressively less light in Nagy’s lab. Sipos says his colleagues “did a really excellent job. It used to be be difficult to make this fungus produce the mushroom.” They succeeded in getting one species—Armillaria ostoyae, also the species of the giant Oregon fungus—to produce mushrooms.

All that trouble paid off though. When the team got the sequencing data back, they noticed that the same networks of genes appear to be active in both the fungus’s rhizomorphs and its mushrooms. It suggests one potential evolutionary origin for rhizomorphs in this genus: Armillaria could have gained its rhizomorphs—and consequently its ability to spread so wide—by co-opting genes originally used to grow mushrooms. Nagy speculates the rhizomorphs may be akin to mushroom stems that failed to sprout and grow a cap, instead growing long and thin underground.

The rise of Armillaria has come at the expense of trees. The fungus actually grows into trees and spread under the bark. At first they digest living wood and when they’ve done enough damage, they continue to feast on the dead wood. “You can basically see entire hills wiped out, entire forests wiped out,” says Nagy. You can’t see much of the humongous fungus in the Malheur Forest in Oregon since the Armillaria is mostly underground, but you can see all the trees it has killed.

Dying conifer forest damaged by Armillaria in Siberia (Courtesy of Igor Pavlov)

Armillaria as a genus not a particularly picky eater either, and it attacks all sorts of plants. Understanding how the fungus spreads could impact many agricultural industries. For example, Kendra Baumgartner, a plant pathologist at the U.S. Department of Agriculture, studies Armillaria that specifically attack California vineyards. She was ecstatic to see the new study, which also catalogues the genomes, proteins, and active genes in Armillaria. “They generated an incredible amount of data,” she says. When we spoke last week, she told me she had the article’s official publication date marked on her calendar, so she could start digging through the data as soon as it’s out.

This fall, Anderson went back to the Michigan fungus whose discovery he first reported in the 1990s. He is also sequencing this fungus now—specifically, different parts of the same fungus—in hopes of understanding how it has mutated over its 1,500 years of life. DNA-sequencing technology has come a long way in 25 years.

But there is another thing Anderson told me he wishes he could do that he knows he never will. For all the estimates of how big Armillaria can grow, no one has really seen it in full. “I wish all of the substrate”—the soil and matter the fungus grows in—“would be transparent for five minutes, so I could see where it is and what it’s doing. We would learn so much from a five-minute glimpse.”