People have known about the "honey mushroom" for some time, but were not aware of how large and invasive this species of fungus could be. The fungus was investigated more closely by researchers when they realized that it was responsible for killing large groves of evergreen trees. When foresters cut into an infected tree they would find spreading white filaments, mycelia, which draw water and carbohydrates from the tree to feed the fungus. Researchers collected samples of the fungus from a widespread area and analyzed the DNA. A large sample of the specimens they collected turned out to be from a single organism.

Until August of 2000 it was thought that the largest living organism was a fungus of the same species (Armillaria ostoyae) that covered 1,500 acres (600 hectares) found living in the state of Washington. But then mycology experts surmised that if an Armillaria that large could be found in Washington, then perhaps one just as large could be responsible for the trees dying in the Malheur National Forest in the Blue Mountains of eastern Oregon. Researchers were astonished at the sheer magnitude of the find. This most recent find was estimated to cover over 2,200 acres (890 hectares) and be at least 2,400 years old, possibly older.