Armillaria is a genus of fungi that includes the A. mellea species ('honey fungus') that live on trees and woody shrubs. It includes about 10 species formerly categorized summarily as A. mellea. Armillaria sp. are long-lived and form the largest living fungi in the world. The largest known specimen (an A. ostoyae) covers more than 3.4 square miles (8.8 km2) in Oregon and is estimated to be 2,500 years old. Some species of Armillaria display bioluminescence. Armillaria can be a destructive forest pathogen. It causes "white rot" root disease. As it feeds on dead plant material, it can kill its host with little negative effect to itself.

Closest relatives