Armillaria ostoyae

Armillaria ostoyae (synonym A. solidipes) is a pathogenic species of fungus in the family Physalacriaceae. It has decurrent gills and the stipe has a ring. The mycelium invades the sapwood of trees, and is able to disseminate over great distances under the bark or between trees in the form of black rhizomorphs ("shoestrings"). In most areas of North America, it can be distinguished from other Armillaria species by its cream-brown colors, prominent cap scales, and a well-developed ring.

The species grows and spreads primarily underground, such that the bulk of the organism is not visible from the surface. In the autumn, the subterranean parts of the organism bloom "honey mushrooms" as surface fruits. Low competition for land and nutrients often allow this fungus to grow to huge proportions, and it possibly covers more total geographical area than any other single living organism. It is common on both hardwood and conifer wood in forests west of the Cascade Range in Oregon.

A spatial genetic analysis estimated that an individual specimen growing over 91 acres (37 ha) in northern Michigan weighs 440 tons (4 x 10⁵ kg). Another specimen in northeastern Oregon's Malheur National Forest is possibly the largest living organism on Earth by mass, area, and volume; it covers 3.5 square miles (2,200 acres; 9.1 km²) and weighs as much as 35,000 tons (about 31,500 tonnes).