Armillaria ostoyae

Armillaria ostoyae is a fungus commonly known as a Honey mushroom, and sometimes called Shoestring Rot.

This is the most common variant in the western U.S., 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 mushroom of this type in the Malheur National Forest in the Blue Mountains of eastern Oregon, U.S. was found to be the largest fungal colony 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. While an accurate estimate has not been made, the total mass of the colony may be as much as 605 tons. If this colony is considered a single organism, then it is the largest known organism in the world by area, and rivals the aspen grove "Pando" as the known organism with the highest living biomass.

In 1992, a relative of the Blue Mountains clone was discovered in southwest Washington state. It covers about 6 km² (1500 acres).

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.