If you were asked to think of the world's largest organism, you might come up with a whale of some sort, maybe an elephant. If you're a bit of a trivia buff, you might come up with Pando, a colony of aspen trees in Utah that all share the same root system.

None of these answers are incorrect, but there may be one organism on the planet larger than even Pando. It's a single growth of the fungus Armillaria ostoyae, and if you ever visit Oregon's Malheur National Forest, it could be right under your feet.

Referred to as "the humongous fungus," this A. ostoyae growth covers at least 482 acres and is estimated to be between 1,900 and 8,650 years old. (Pando may be older at 80,000 years, but it only covers about 106 acres.) However, since the A. ostoyae growth is almost entirely underground, it could be even larger than we realize, but without transparent soil, it's hard to know. We're able to identify Armillaria because the fungus grows not only mushrooms, but it also grows thick, rope-like rhizomorphs that extend underground as it seeks out trees to feast upon.

What may not be a mystery any longer, however, is that scientists think they know how an A. ostoyae growth could get so big in the first place.

Tendrils though the forest

Unidentified Armillaria rhizomorph growing on a tree Armillaria rhizomorphs can have a noticeable effect on a forest. (Photo: Ericsteinert/Wikimedia Commons)

A study published in the journal Nature Ecology & Evolution sequenced and analyzed four Armillaria species in an effort to see what made them tick. This involved growing the Armillaria species in a lab, using either rice, sawdust, tomatoes or "orange media." The Armillaria grew their rhizomorphs without any prompting by researchers, but to get mushrooms for comparisons, they had to slowly move the samples to colder and less well-lit areas of the lab to mimic the onset of fall, when the mushrooms sprout.

What the researchers found was that the rhizomorphs and the mushrooms shared the same type of active gene network. What this potentially means is that the Armillaria species' ability to grow rhizomorphs may have come directly out of using the genes it uses to create mushrooms. Speaking to the Atlantic, one of the researchers, László Nagy of the Hungarian Academy of Sciences, said the rhizomorphs might be similar mushroom stems that simply failed to sprout and instead grew underground, spreading as quickly as mushrooms often do.

Greedy fungi

Armillaria ostoyae mushrooms grow on a tree Armillaria ostoyae mushrooms may be the reason why the fungus is able to grow rhizomorphs. They're also the most visible sign of the fungus. (Photo: Nadzin/Shutterstock)

But being underground creates problems for the forest. The Armillaria rhizomorphs evolved certain functions over time, some of which are associated with the spreading of disease. In this case, it's called white rot. The rhizomorphs, thanks to "diverse gene repertoires" have a number of genes that contribute to causing cell deaths in plants. On average, Armillaria rhizomorphs had 669 small secreted proteins that signal pathogenic interactions, compared to the 552 of such proteins found in other tested saprotrophs. Such a diverse set of genes provides the Armillaria with a possible advantage when it comes to beating competitor microbes to untouched and healthy root systems. This lack of competition, in turn, may allow the Armillaria to grow as far and wide as it does.

In the case of the humongous fungus in Malheur National Forest, A. ostoyae and its rhizomorphs are responsible for killing plenty of trees. According to the U.S. Forest Service, the symptoms of Armillaria are often striking. Living trees will have sparse, yellow-green foliage and resin exuding from their bases. Dead trees will suffer a loss of branches and tree bark. Worse still is that many trees will remain standing even after death, sometimes taking years to topple. All the while, the rhizomorphs keep feeding, regardless of if the tree is alive or dead. So while you may not be able to see the world's largest organism, you can certainly see the effects it has on its environment.

There may be some light at the end of this tunnel, however. Nagy and his team's study is such a treasure trove of information that it could lead to other researchers developing strategies to contain the spread and damage caused by Armillaria.