Armillaria ostoyae (synonym Armillaria solidipes) is a species of plant-pathogenic fungus (mushroom) in the family Physalacriaceae. It is the most common variant, in the western United States, of the group of species under the name Armillaria mellea. Armillaria ostoyae is common on both hardwood and conifer wood in forests west of the Cascade Range in Oregon, United States. It has decurrent gills and the stipe has a ring. The mycelium invades the sapwood 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, Armillaria ostoyae can be separated from other species by its physical features: cream-brown colors, prominent cap scales, and well-developed stem ring distinguish it from any Armillaria.