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Xerocomellus atropurpureus

by Michael Kuo, 1 April 2024

One of two phylogenetic species from western North America that used to be called "Boletus zelleri," Xerocomellus atropurpureus is gorgeous when young and fresh, featuring a dark brown cap, a red and yellow stem, and a yellow pore surface. With age, it begins to lose some of its pizzazz, but still retains much of its original glory.

Separating Xerocomellus atropurpureus from Boletus Xerocomellus zelleri is not for the faint of heart. The two species are clearly separated by their ITS and LSU sequences, but both appear to be mycorrhizal with conifers, and their physical features overlap. The cap of atropurpureus, when young, is often tightly wrinkled, and this may be the most useable separator. Several other West-Coast species are also similar, but more easily separated by their association with hardwoods, and/or their consistently mosaic-like, cracked caps, and/or their strong blue staining; see Xerocomellus diffractus for a run-down.

Description:

Ecology: Mycorrhizal with conifers; growing alone, scattered, or gregariously; fall and winter; originally described from Oregon (Frank et al. 2020); distributed in the Pacific Northwest and northern California, and in the Sierra Nevada and Cascade ranges. The illustrated and described collections are from California.

Cap: 4–9 cm; convex, becoming broadly convex; dry; often tightly wrinkled when young; bald or, when young, with a whitish bloom; sometimes developing fissures in age, but not typically becoming finely cracked; dark brown to nearly black, often with reddish to purplish hues.

Pore Surface: Yellow when young, becoming olive yellow; not bruising, or bruising slowly bluish gray to brownish; with 1–2 angular pores per mm; tubes to 6 mm deep.

Stem: 4–7 cm long; 0.5–2 cm thick; at first swollen centrally or basally, but more or less equal with age; solid; red overall, or yellow (especially apically) with red areas; sometimes bluing slowly when bruised; basal mycelium yellowish.

Flesh: Whitish to yellowish in cap; yellowish to yellow in stem; not staining when sliced, or staining slowly and erratically bluish.

Odor and Taste: Not distinctive.

Chemical Reactions: Ammonia negative on cap and flesh. KOH negative to grayish on cap; orangish to dark yellow on flesh. Iron salts negative on cap and flesh.

Spore Print: Olive brown.

Microscopic Features: Spores 12–16 x 4–6 µm; boletoid-fusiform; smooth; yellowish to golden in KOH; inamyloid. Basidia 35–38 x 5–8 µm; clavate; mostly 4-sterigmate. Hymenial cystidia not found. Pileipellis a trichoderm or palisadoderm; elements 4–15 µm wide, subglobose to ellipsoid or elongated-cylindric; smooth or a little encrusted; yellowish to brownish in KOH; terminal cells varying (sometimes within the same cap) from cylindric-elongated with apices subclavate or merely rounded, to subglobose, ellipsoid, or awl-shaped.

REFERENCES: J. L. Frank, N. Siegel, & C. F. Schwarz, 2020. (Smith, 1949; Thiers, 1975; Smith, Smith & Weber, 1981; Phillips, 1991/2005; Schalkwijk-Barendsen, 1991; Lincoff, 1992; Both, 1993; Bessette, Roody & Bessette, 2000; Miller & Miller, 2006; Kuo, 2007; Trudell & Ammirati, 2009; Siegel & Schwarz, 2015; Frank et al., 2020.) Herb. Kuo 01130509, 01141501.

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Spores

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Cite this page as:

Kuo, M. (2024, April). Xerocomellus atropurpureus. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/xerocomellus_atropurpureus.html