Armillaria solidipes (formerly Armillaria ostoyae)
It is known to be one of the largest living organisms, where scientists have estimated a single specimen found in Malheur National Forest in Oregon to have been growing for some 2,400 years, covering 3.4 square miles. Armillaria solidipes grows and spreads primarily underground and the bulk of the organism lies in the ground, out of sight. Therefore, the organism is not visible to anyone viewing from the surface. It is only in the autumn when this organism will bloom “honey mushrooms”, visible evidence of the organism lying beneath. Low competition for land and nutrients have allowed this organism to grow so huge and become arguably the largest living organism
Armillaria solidipes is mostly common in the cooler regions of the northern hemisphere. In North America, this fungus can be found on host coniferous trees located in the northwestern forests of the continent in British Columbia and the Pacific Northwest. While Armillaria solidipes is distributed throughout the different biogeoclimatic zones of British Columbia, the root disease causes the greatest amount of problem in the interior parts of the region in the Interior Cedar Hemlock (ICH) biogeoclimatic zone. It is both present in the interior where it is more common as well as along the coastal lines -- In most areas of North America, Armillaria solidipes can be separated from other species by its physical features. Its brown colors, fairly prominent scales featured on its cap, and the well-developed ring on its stem sets it apart from any Armillaria
A mushroom of this type in the Malheur National Forest in the Strawberry Mountains of eastern Oregon, U.S. was found to be the largest fungal colony in the world, spanning 8.9 square kilometres (2,200 acres) of area. This organism is estimated to be 2,400 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 Strawberry Mountains clone was discovered in southwest Washington state. It covers about 6 square kilometres (1,500 acres). Another "humongous fungus" is a specimen of Armillaria gallica found at a site near Crystal Falls, Michigan, which covered 0.15 square kilometres (37 acres).