Armillaria is a genus of fungi that includes the A. mellea species known as honey fungi that live on trees and woody shrubs. It includes about 10 species formerly categorized summarily as A. mellea. Armillarias are long-lived and form the largest living organisms in the world. The largest known organism (of the species Armillaria ostoyae) covers more than 3.4 square miles (8.8 km2) in Oregon's Malheur National Forest and is between 1900 and 9600 years old. Some species of Armillaria display bioluminescence, resulting in foxfire.
Armillaria can be a destructive forest pathogen. It causes "white rot" root disease (see Plant pathology section) of forests, which distinguishes it from Tricholoma, a mycorrhizal (non-parasitic) genus. Because Armillaria is a facultative saprophyte, it also feeds on dead plant material, allowing it to kill its host, unlike parasites that must moderate their growth to avoid host death.
In the Canadian Prairies (particularly Manitoba), Armillaria is referred to often as openky (Ukrainian: опеньки), meaning "near the stump" in Ukrainian.