Armillaria ostoyae  is a species of fungus in the Physalacriaceae family. It 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").
In most areas of North America, Armillaria ostoyae can be separated from other species by its physical features. Its brown colors, fairly prominent scales featured on its cap, and the well develop ring on its stem sets it apart from any Armillaria.
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 ostoyae 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.
The species was formerly known as Armillaria ostoyae Romagn., until a 2008 publication revealed that the species had been described under the name Armillaria solidipes by Charles Horton Peck in 1900, long before Henri Romagnesi had described it in 1970.
This fungi, like most parasitic fungi, reproduce sexually. The fungi begins their life as spores, released into the environment by a mature mushroom. In the specific case of Armillaria ostoyae, it has a white spore print in which all the spores are stored until they are released and dispersed. There are two types of mating types for spores (not male and female but similar in effect). The spores can be dispersed by environment factors such as wind or they can be redeposited by an animal. Once the spores are in a resting state, the single spore must come in contact with a spore of an opposite mating type and of the same species. If the single spore isolates are from different species, the colonies will not fuse together and they will remain separate. When two isolates of the same species but different mating types fuse together, they soon form coalesced colonies which become dark brown and flat. With this particular fungi it will produce mycelial cords also known as rhizomorphs. These rhizomorphs allow the fungi to obtain nutrients from long distances away. These are also the main factors to its pathogenicity. As the fruiting body continues to grow and obtaining nutrients, it forms into a mature mushroom. Armillaria ostoyae in particular grows a wide and thin sheet-like plates radiating from the stem which is known as its gills. The gills hold the spores of a mature mushroom. This is stained white when seen as a spore print. Once spore formation is complete, this signifies a mature mushroom and now is able to spread its spores to start a new generation.