BOLETUS OROVILLUS Thiers and Kowalski ex Thiers, Mycologia 58:825. 1966

Pileus 10-15 cm broad at maturity, convex to broadly convex to plano-convex, surface moist, becoming distinctly viscid, subtomentose to velutinous to appressed fibrillose, with age becoming rimose-areolate, particularly on the disc; color during all stages of development a very intense yellow ("picric yellow" to "strontian yellow" to "empire yellow"), slightly darker and duller on the disc with age; margin strongly incurved, becoming decurved, entire. Context up to 2 cm thick, yellow, bluing slightly when exposed. Taste and odor mild.

Tubes 0.5-1.5 cm long, adnate, becoming depressed at maturity, yellow, unchanging when exposed; pores very small, one to two per millimeter, bright red when fresh and with red exudate, angular, unchanging when bruised.

Stipe 7.5-12.5 cm long, 2-5 cm thick at the apex, ventricose to clavate, sometimes bulbous at the base, yellow mycelium at the base, solid; surface dry, smooth, not reticulate, glabrous to appressed fibrillose to more or less granulose at the apex, more or less concolorous with the pores in apical portion, becoming yellow toward the base. Context yellow, changing to pale blue when exposed.

Spore print color unknown. Spores 5.5-6.4 X 3-4 µm pale ochraceous in Melzer's, hyaline to subhyaline in KOH, ellipsoid to subovoid to occasionally narrowed in median portion and somewhat enlarged at the ends, smooth, thin-walled.

Basidia 30-39 X 8-10 µm hyaline, clavate, four-spored. Hymenial cystidia 35-47 X 9-14 µm scattered to numerous, subclavate to mucronate to somewhat contorted, thin-walled; ochraceous tawny to cinnamon brown in KOH.

Tube trama divergent from a distinct central strand that stains pale ochraceous in KOH. Pileus trama amyloid, interwoven, homogeneous, hyaline in KOH. Pileus cuticle differentiated as a trichodermium of tangled, more or less interwoven, narrow hyphae, which appear subgelatinous in KOH. Stipe cuticle differentiated as a layer of interwoven hyphae. Clamp connections absent.

Chemical reactions KOH-cuticle red; HCl-cuticle red at first, then blackening; HNO₃-cuticle red, then blackening; sulfoformalin-cuticle red, then blackening.

Habit, habitat, and distribution Solitary in humus under digger pine (P. sabiniana Dougl.). It is known only from the type locality, which is in the vicinity of Oroville. Apparently very rare. Only other known collection is from near Olympia, Washington.

Material studied Butte County: Kowalski 895.

Observations This is one of the most distinctive boletes in California. The intensely yellow pileus, red pores, smooth, nonreticulate stipe, and small spores are the major distinguishing features. It apparently belongs in the subsection Luridi but it is very difficult to determine its relationship among species in that section. The yellow pileus and small spores are features of the subsection Sulphurei of the section Subtomentosi; however, no species known in that section has red pores and most are lignicolous.

Edibility undetermined, but it should be avoided.