A large concern in Japan is root rot experienced in conifer species as a result of Armillaria, a group of fungus species found in Asia.
A preferred host of gypsy moth, death occurs when nearly complete defoliation by gypsy moths is followed by a fungal infection by Armillaria spp.
It is known to be one of the largest living organisms, where scientists have estimated a single specimen found in Malheur National Forest in Oregon to have been growing for some 2,400 years, covering 3.4 square miles (8.4 km²) and colloquially named the "Humongous Fungus." Armillaria solidipes grows and spreads primarily underground and the bulk of the organism lies in the ground, out of sight.
It has been shown to provide protection against such pathogens as Rhizoctonia, Pythium and even Armillaria.
Old trees are commonly attacked by various fungal disease and decay species, notably Heterobasidion annosum and Armillaria species, which rot the heartwood and eventually leave the tree liable to windthrow, and Rhizina undulata, which may kill groups of trees following minor grass fires that activate growth of the Rhizina spores.