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Humongous Fungus


Most people think the blue whale is Earth’s largest known organism, but it isn’t. It’s a fungus! Oregon’s Malheur National Forest is home to a very large growth of fungus called Armillaria ostoyae.  Discovered in 1998 and nicknamed the Humongous Fungus, it is believed to be Earth’s largest single living organism. It is estimated to weigh as much as 35,000 tons and may be as old as 8,650 years, making it a contender for oldest living organism too! This fungus covers 2,385 acres in Oregon’s Blue Mountains.  

Armillaria ostoyae growing on log


This fungus grows underground, spreading via hyphae, fine filaments that mat together and excrete digestive enzymes. But Armillaira also has unique rhizomorphs, which are long black filaments of fungal tissue that grow bridge-like structures under the soil to infest trees. The fungus feeds on these trees. Eventually the tree’s roots decay and the tree dies. This is a very slow-spreading fungus, only extending 0.7 to 3.3 feet per year, so it can take 20 to 50 years for the fungus to kill a tree! It then continues to feed on the dead wood of the tree once it has died.  

In the Malheur Forest, there are actually five independent, genetically distinct occurrences of Armillaria ostoyae, each ranging from 50- 2,385 acres. In this region, the most commonly infected tree is the grand fir. Armillaria produces growths in the fall, commonly known as honey mushrooms. They grow in clumps out of the base of infected trees. They are about 2-4 inches tall, with brown gills, and have caps between 2- 5 inches across.  

Armillaria ostoyae is common in the Pacific Northwest, where it has perfect growing conditions, but it can also be found in Europe, Asia, and other parts of North America.  

Being this incredibly large is surprisingly normal among fungi, but the humongous fungus is the record holder for now!