Mycologia 106: 475. 2014.
Common Name: butter bolete
Misapplied name: Boletus regius Krombholz
Cap 8.0-15.0 (20) cm broad, strongly convex, expanding to plano-convex; margin incurved, becoming decurved to plane in age; surface subviscid when moist, otherwise dry, with irregular bumps, pits and depresssion, at first ornamented with patches of appressed fibrils, these often wearing away in age, then appearing glabrous; color rose-red to pale-pink, quickly fading with exposure to light to various combinations of buff, tan, or dull pale-yellow; context lemon-yellow, firm, up to 2.5 cm thick, slowly bluing when cut or injured, bluing reaction strongest just above the tube layer; odor mild; taste mild, pleasant.
Pores lemon-yellow when young, 2-3 per mm, maturing to dingy-yellowish with a slight olivaceous tint, 1-2 per mm in age, bluing when bruised; tubes adnate in attachment, becoming depressed in age, up to 2.5 cm long, pale-yellow, bluing like the pores.
Stipe 5.0-9.0 cm long, 3.0-4.0 cm thick at apex, clavate to sub-bulbous, solid, fleshy; surface of upper half conspicuously reticulate, the ornamentation and background color yellowish, sometimes becoming pallid, occasionally bluing when young; cortex yellowish, otherwise white, unchanging, often reddish at the base; partial veil absent.
Spores 13.0-15.5 x 4.0-5.0 µm, elliptical to slightly spindle-shaped in face-view, smooth, thin-walled; spore print olive-brown.
Solitary, scattered to clustered, spring and fall under conifers in the Sierra, and in mixed confer-hardwood forests along the coast during the fall, possibly also in the spring.
Edible and excellent.
Butyriboletus autumniregius is a beautiful, robust bolete with an aspect similar to Boletus edulis. A notable difference, however, is that Butyriboletus autumniregius has a rose-pink cap while Boletus edulis has a brownish cap. Additionally, the pore surface and stipe reticulation of Butyriboletus autumniregius are yellowish, compared to pallid in Boletus edulis. A final distinction is the tendency for the pores of Butyriboletus autumniregius to blue, a feature absent in Boletus edulis. Spring fruitings in the Sierra of a similar mushroom is Butyriboletus primiregius. They should be compared with Boletus rex-veris, a close relative of Boletus edulis, and the occasional fruiting of Caloboletus rubripes. Boletus rex-veris has a somewhat similarly colored cap when young, pinkish-brown to dull rusty-brown, but the pore surface in youth is pallid not lemon-yellow and the stipe reticulation light-brown over a cream-colored background. Caloboletus rubripes is easily distinguished by the lack of a reticulated stipe and a bitter taste. In coastal forests Butyriboletus autumniregius is often confused with a close relative, Butyriboletus persolidus. The latter, however, has a rusty-brown to yellowish-brown cap as opposed to rose-pink in Butyriboletus autumniregius.
Arora, D. & Frank, J.L. (2014). Clarifying the butter Boletes: a new genus, Butyriboletus, is established to accommodate Boletus sect. Appendiculati, and six new species are described. Mycologia 106(3): 464-480.
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Desjardin, D.E., Wood, M.G. & Stevens, F.A. (2015). California Mushrooms: The Comprehensive Identification Guide. Timber Press: Portland, OR. 560 p.
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Thiers, H.D. (1975). California Mushrooms—A Field Guide to the Boletes. Hafner Press: New York, NY. 261 p. (WWW)