Summary:
Armillaria ostoyae is a member of the Armillaria mellea group. It is a pathogen of conifers. Features within the group include brown cap covered by dark scales, sometimes large size, clustered cespitose habit on stumps and at the base of coniferous and hardwood trees, belt-shaped rhizomorphs, and clamp connections on basidia as well as binucleate subhymenial cells. A single mycelium of this species is said to cover 2200 acres (880 hectares) in eastern OR, (Sept(1)). Armillaria solidipes Peck Bull. Torrey Bot. Club 27(12): 611. 1900 is the correct name according to Burdsall(8) who re-examined that type, but Hunt(1) present evidence to the contrary. Armillaria obscura is a misapplied name according to Termorshuizen and Arnolds (1987). Armillaria ostoyae is common in the Pacific Northwest.

Cap:
5-10cm across, convex to nearly flat; "tan to yellowish brown to more typically dark reddish brown"; "densely covered with dark reddish brown to blackish hairs", dry, (Bessette), 5-10cm across, hemispheric - bell-shaped or obtusely parabolic, then convex and finally flat, margin inrolled then downturned; sometimes of pale colors like 'orange white' or 'caramel brown', but usually darker like reddish brown, 'raw sienna', 'light brown', yellowish brown, 'burnt Sienna', 'tan', or 'leather', and even very dark colors like 'eye brown', 'brownish gray', with distinct scales in shades of dark reddish colors ranging from grayish red, 'liver (brown)', 'photo brown', or in shades of dark brownish color like 'caramel (brown)', 'beaver', 'leather', 'eye brown', to sometimes 'anthracite' and black, distributed over the surface but more densely toward center, margin usually colored as cap but sometimes paler; dry, (Berube), up to 30cm across, (Volk), the scaliest of the Armillaria species, (Kibby)

Flesh:
firm, rather thick at center; white, (Bessette), firm 1-2cm thick at top of stem; white, in stem white to 'alabaster', (Berube)

Gills:
attached to subdecurrent [somewhat decurrent], close; "white to cream at first, becoming grayish orange to cinnamon", (Bessette), adnate to slightly decurrent becoming sinuate in mature specimens, thick at the point of attachment to the stem but thinner toward the margin; white or ''cream'' when young, to grayish orange, ''cinnamon'' later, (Berube)

Stem:
5-20cm long and about 1.5cm wide at top, typically widening downward at first becoming equal, "often quite tapered at very base, fibrous, the fibers generally orangish to reddish brown", entire stem staining mahogany to blackish, often with adhering bits of partial veil, with yellow mycelium at the extreme base; rhizomorphs flattened, (Bessette), 5-20cm long, about 1.5cm wide at top, club-shaped when young, becoming more or less equal, fibrous; covered with fibers ranging from grayish orange, ''light brown'', ''burnt Sienna'' and ''hazel'', staining black or sometimes ''mahogany (red)''; sometimes deeply striate; often with bright yellow to greenish yellow mycelial fibers at base only, (Berube), black rhizomorphs are branching and flattened with blunt tips, (Kibby)

Veil:
"partial veil thick, membranous, leaving a whitish ring with a fluffy brown margin", (Bessette), thick membranous ring, usually white to ''alabaster'', circled with a thick, fluffy band at margin ranging from "tan" to "chestnut", broken in many places leaving the stem marked and covered at many locations with dark fluffy remnants concolorous to the ring, (Berube), ring white with blackish brown scales on the edge and down the stem in many specimens, (Kibby)

Odor:
not distinctive (Bessette)

Taste:
not distinctive (Bessette)

Microscopic spores:
spores 8-11 x 5.5-7 microns, broadly elliptic to oval, smooth, inamyloid, colorless; clamp connections at the bases of some basidia, (Bessette), spores 8-11 x 5.5-7 microns, broadly elliptic to ovate, smooth, inamyloid, apiculate; basidia 4-spored, some with a clamp connection; pleurocystidia absent, margin of gills composed of polymorphic sterile cells; subhymenial hyphae filamentous, binucleate, unclamped [but later in the same article says A. ostoyae has clamp connections in the subhymenium], (Berube)

Spore deposit:
pale cream (Bessette), ivory (Berube)

Notes:
Collections were examined from BC, QC, ON, and A. ostoyae has a very wide distribution throughout North America, from Pacific to Atlantic coast but predominantly "in the northern temperature region", including BC, WA, ON, QC, ME, MI, NH, NY, VT, Denmark, Finland, France, Germany, Norway, Poland, and United Kingdom (England, Scotland), (Berube). It was reported from ID by Andrew Parker, pers. comm. and presumably occurs in OR.

EDIBILITY

a fine edible if thoroughly cooked (Bessette), has been reported to cause some people some gastrointestinal upset if collected from hemlock, but Volk believes most of the reported upset is due to undercooking (and overeating) of larger collections, (Volk), there are reports of toxic reactions to Armillaria ingestion - gastrointestinal symptoms dominate the clinical picture, but hallucinations have been reported in a few cases (Benjamin)

Synonyms and Alternate Names:
Armillaria obscura (Secr.) Herink misapplied name
Armillaria solidipes (Romag.) Herink
Armillariella ostoyae Romagn.