When asked about the largest living organism on Earth, most people would reply, “The blue whale.” However, this is not entirely true. Yes, the blue whale is the largest living animal, but it isn’t the largest creature. The largest living organism on Earth is actually something you would never think of: a giant fungus! The Armillaria ostoyae (A. ostoyae), or the Humongous Fungus, covers 2,385 miles (about four square miles) of the Malheur National Forest in Oregon.

This mushroom belongs to the Armillaria genus, popularly known as the honey fungus. When they colonize, they leech off and kill the roots of trees and woody plants, causing them to decay. In a large forest, the Armillaria ostoyae can grow to be quite expansive at the cost of a few thousand acres of trees. 

The various yellow-brown mushrooms seen on the surface are grown in large clumps. These mushrooms are produced annually around the base of infected or newly-killed trees. Although they may look small, these fungi are actually the fruiting bodies of much larger organisms. They consist of black rootlike aggregations of hyphae called rhizomorphs, which search for new hosts. The fungus has a large underground network of roots called mycelia. These roots are spread throughout the forest floor. 

The reason why the fungi are able to grow to such an enormous size was discovered fairly recently. According to BBC.com, “In 1988, a team from the U.S. Forest Service set out to investigate the cause of large tree die-offs in the Malheur National Forest, in east Oregon.” The team identified numerous affected areas with aerial photographs. They also collected root samples from 112 dead and dying trees. Tests were conducted, which showed that all but four of the trees had been infected with the honey fungus. The researchers found that when mycelia from multiple genetically identical A. ostoyae meet, they can fuse to form one individual. The researchers used this ability to grow fungi samples in pairs in Petri dishes. They observed the ones that fused and the ones rejected one another, concluding that 61 of the trees had been struck down by the same clonal colony, individuals with an identical genetic make-up, all originating from one organism.

According to Forbes.com, the honey fungus “poses a challenge to those managing threatened ecosystems.” It also threatens the timber industry. Economic loss is a result of tree death from the honey fungus. Although the fungus can grow expansively and act as a beautiful tourist destination, it is very hard to eradicate and can threaten commercial industries.

Photo courtesy of BIBALAX.ORG