Armillaria ostoyae is the binomial name for one species of fungus commonly known as a "Honey mushroom", and sometimes called "Shoestring Rot". The term humongous fungus is also applied to this species.

This is the most common variant in the western US, of the group of species that all used to share the name, Armillaria mellea. Armillaria ostoyae is quite common on both hardwood and conifer wood in forests west of the Cascade crest. The mycelium attacks the sapwood and is able to travel great distances under the bark or between trees in the form of black rhizomorphs ("shoestrings").

A population of mushrooms in the Malheur National Forest in the Blue Mountains of eastern Oregon, USA was found to the largest single organism in the world, spanning 8.9 km² (2200 acres) of area. This organism is estimated to be 2400 years old. The fungus was written about in the April 2003 issue of the Canadian Journal of Forest Research.

The Blue Mountains clone has a relative that covers about 6 km² (1500 acres) in southwest Washington state that was discovered in 1992.

Another "humongous fungus" is a specimen of Armillaria bulbosa found at a site near Crystal Falls, Michigan covers 0.15 km² (37 acres), and was published in Nature 356:428-431.

External links

• Possible 1000-year-old in Switzerland
• BBC News report about the largest Armillaria in the world
• Mycologist's site about giant Armillaria in Michigan's Upper Peninsula