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Species Information
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.
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.
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)
Habitat / Range
typically in large clusters but sometimes single on or about stumps or trees, July to November, (Bessette), in BC found from 49 degrees latitude to 53 degrees primarily on conifers, but hardwoods within disease centers in conifer stands were frequently attacked and killed, (Morrison), on the Olympic peninsula in Washington, almost exclusively on conifers, especially Tsuga heterophylla (Western Hemlock), but also Pseudotsuga menziesii (Douglas-fir) and Picea sitchensis (Sitka spruce), (Banik), singly but most commonly in large fasciculate groups on decaying wood debris, dead stumps, dead trees, dead roots, declining trees, and healthy roots, hosts include Acer (maple), Betula (birch), and Quercus (oak), (Berube), a serious problem in Malus and Prunus orchards in Michigan (Burdsall), usually in clusters, mostly on conifers, but also on hardwoods and shrubs such as Salix (willow) and Rubus spectabilis (salmonberry), (Trudell), summer, fall
Similar Species
Armillaria sinapina has 1) a pale to dark brown cap with reddish tinges, 2) a mustard yellow veil [but see Volk comment below], 3) brown scales, (as opposed to dark to very dark brown cap densely covered with dark scales), 4) fibrous, thin, appressed annulus that is mustard to golden yellow (as opposed to thick, membranous, white and brown), 5) club-shaped stem covered with mustard yellow remnants of veil (as opposed to cylindric, broad and yellow at apex), and 6) cylindrical rhizomorphs with monopodial branching (as opposed to belt-shaped with dichotomous branching), (Berube). It is difficult to distinguish between A. ostoyae and A. sinapina in a colonized stump because both produce white mycelial fans in the bark and cambial zone, however, A. sinapina produces an extensive network of monopodially-branched rhizomorphs while A. ostoyae produces small amounts of dichotomously branched ones; also the fruiting bodies of A. sinapina are usually darker in color, with darker cap scales, smaller, and more numerous than those of A. ostoyae, (Allen). The golden yellow color of the universal veil has not been observed on western specimens of A. sinapina, (Volk). A. sinapina differs only slightly in appearance, with a cobwebby veil and slightly smaller cap scales, growing singly or in clusters of only a few individuals, (Trudell). Armillaria gallica has white cobwebby veil, pinkish brown color, bulbous-based stem, and occurs singly or in groups rather than clusters, on or near logs, stumps, or bases of hardwoods such as willow, (Trudell). Armillaria nabsnona 1) has a more orange coloration when fresh, 2) lacks scales on the cap (but small black hairs may be found), 3) has a narrower stem in comparison to the size of the cap, the stem being darker, especially when dried, 4) fruits with hardwoods red alder, usually singly or in groups rather than clusters, and 5) microscopically has a distinctive pattern of branching of the basidia. Armillaria mellea [not recorded from Pacific Northwest] can occur in the same habitat, but A. ostoyae distinguished by its nonviscid, predominantly brown to dark brown cap covered by numerous dark scales, the brownish ring (annulus) on the stem, and the clamp connections at the base of the basidia, (Berube).
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)
