E-Flora BC: Electronic Atlas of the Flora of British Columbia

Summary:
Features include dark brown to chestnut brown, dry, fibrillose cap, yellow pores, and brown reticulate stem.

The description is derived from Thiers except where indicated. Boletus fibrillosus is "recorded from the Pacific Coast of California and the Cascades, distribution limits yet to be established" (Bessette). The type is from CA. There are collections from WA and OR at the University of Washington. Paul Kroeger says Boletus fibrillosus appeared in quantity in BC about 2004, and a collection from 2005 in BC is at the University of British Columbia.

Cap:
6-17cm, convex becoming flat-convex to broadly convex to flat with flared margin, margin incurved at first; brown to dark brown, occasionally with paler buff spots; dry, even or somewhat wrinkled, typically tomentose to velvety, becoming fibrillose, sometimes fibrillose-scaly on disc, (Thiers), "brown, cinnamon-brown to chestnut-brown, or dark brown, sometimes with buff-colored spots", (Bessette), "some shade of dark brown, often with lighter areas"; "typically wrinkled and velvety to distinctly fibrillose", (Trudell)

Flesh:
1.5-3cm thick, soft, floccose [cottony]; white to pale buff, unchanging when exposed, in stem white, unchanging when exposed

Pores:
1-2 per mm, angular; light yellow when young, dark yellow when old, unchanging; tube layer 1.5-2.5cm thick, "narrowly and shallowly depressed" around stem, colored as pores, unchanging or darkening slightly when bruised, (Thiers), light yellow or somewhat darker, tubes similar in color, (Trudell), pale yellow to yellow or dingy olive-yellow (occasionally pallid when very young), (Arora)

Stem:
10-16cm x 2-3cm, equal to club-shaped, rarely narrowing downward, often pinched at base, solid; pale yellow at top, otherwise light brown to brown; dry to moist, reticulate [netted] over most parts, white basal mycelium, (Thiers), long relative to cap, often pinched off at base; pale at top, dark brown in lower part; strongly reticulate, covered by mycelium at base, (Trudell), 8-16cm x 2-4cm, usually lined or fibrillose, reticulate at least at top, (Arora)

Chemical Reactions:
cap cuticle red to pink with application of KOH

Microscopic:
spores 13-17.5 x 3.5-5.5 microns, subfusoid [somewhat spindle-shaped] to ventricose [wider in middle], smooth, pale ochraceous in Melzer''s, pale yellow to colorless in KOH, moderately thick-walled; basidia 4-spored, 35-40 x 8-12 microns, clavate, colorless in KOH; hymenial cystidia scattered, inconspicuous, 38-46 x 6-8 microns, subcylindric to fusoid-ventricose with elongated apices, colorless, thin-walled; cap cuticle differentiated as a trichodermium of hyphae have +/- equal cells; stem cuticle differentiated as a layer of fertile basidia and caulocystidia, which are colorless, subcylindric to clavate, abundant on edges of reticulum

Spore Deposit:
dark olive-brown