Armillaria, is a genus of parasitic 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 some of the largest living organisms in the world. The largest known organism (of the species Armillaria solidipes) covers more than 3.4 square miles (8.8 km2) in Oregons Malheur National Forest and is more than 2,400 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, meaning near the stump in Ukrainian.