The Humongous Fungus
Deep within Oregon's Malheur National Forest, an enormous life form thrives mostly unseen. This organism, a single genetic individual of the fungus Armillaria ostoyae, covers an area of 2,385 acres, or nearly 3.7 square miles. Its discovery in 1998 redefined the scale of life on Earth. Scientists estimate its age to be at least 2,400 years, and possibly as old as 8,650 years. While other organisms are taller or heavier in a single mass, this fungus is the largest known living thing by total area.
The vast majority of the organism exists underground as a network of thread-like filaments called mycelium. This subterranean body spreads through the soil and tree roots. It also forms black, root-like structures called rhizomorphs, which have earned it the nickname "shoestring fungus." These rhizomorphs are its primary tools for spreading over long distances and infecting new trees. The total weight of the fungus is estimated to be as much as 35,000 tons. The only visible evidence of this massive network appears in the fall, when it produces clusters of honey-colored mushrooms at the base of infected trees. These fruiting bodies, the source of the common name "honey mushroom," are a small, reproductive part of the whole.
A Slow and Relentless Parasite
Armillaria ostoyae is a tree pathogen. It causes a condition known as Armillaria root disease in conifers throughout the Pacific Northwest. The fungus uses its rhizomorphs to penetrate the root systems of healthy trees. Once inside, it grows a fan-shaped mat of white mycelium just beneath the bark, which feels like latex paint. This growth effectively girdles the tree, cutting off the flow of water and nutrients from the roots to the crown.
The process is slow, taking anywhere from 20 to 50 years to kill a mature tree. Infected living trees may show signs of distress, such as changing needle color or producing a larger-than-normal crop of cones. The fungus does not stop once its host is dead; it continues to feed on the dead wood as a saprobe. This dual role as both parasite and decomposer makes it a part of the forest ecosystem. The slow spread of the fungus—estimated at one to three feet per year—gradually creates patches of dead and dying trees within the forest.