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This is the most common variant in the western US, of the group of species that all used to share the name, Armillaria mellea. Armillaria ostoyae is quite common on both hardwood and conifer wood in forests west of the Cascade crest. The mycelium attacks the sapwood and is able to travel great distances under the bark or between trees in the form of black rhizomorphs ("shoestrings"). Populations of this species in eastern Oregon and Washington State, and the related Armillaria bulbosa, in Michigan have been found to be huge organisms spread out over 10 acres (40,000 m²) or more area, making them the world's largest organisms; some are estimated to be over 400 years old.