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Species Information
Summary: Features include 1) a dark to light brown to olive or olive gray cap that is tomentose to velvety and when mature has whitish to pale yellow to reddish cracks, 2) whitish to yellow flesh that turns blue slowly to moderately or erratically, 3) yellow to greenish pores that become large and irregular and typically stain blue slowly to moderately where bruised, 4) a yellowish to golden stem streaked with red punctations especially on lower stem, 5) fruiting under both conifers and hardwoods, and 6) non-truncate spores. Xerocomellus diffractus is fairly common in the Pacific Northwest. It has been identified in the past as Boletus chrysenteron which is a European species. A sister species to Xerocomellus diffractus is indicated by molecular data from a Xerocomellus chrysenteron-like taxon in eastern North America but is not yet named (Frank(9)).
Distribution is from central CA through the Pacific Northwest into BC, east to the Rocky Mountains of WY, and south into AZ; collections were examined from WA, OR, AZ, CA, and WY (Frank(9)). A. Parker, pers. comm., identified what is likely to be this species from ID (under the misapplied name Boletus chrysenteron before Xerocomellus diffractus was published).
Cap: 3-10(14)cm across, "rounded, bun-shaped to broadly convex at first, becoming flat", occasionally slightly wavy when old; "olive, olive gray to leather brown or tan", becoming paler when old, "more rarely reddish to pinkish brown"; in cracks whitish to pale yellow, becoming pinkish when old; "dry, finely velvety, becoming cracked", (Frank), 6-8cm, convex to broadly convex to flat, margin downcurved; dark brown or dark gray, pallid in cracks but usually with reddish tints in cracks toward margin when older; dry, dull, velvety to tomentose, usually shallowly and conspicuously areolate [cracked like dried mud] when old, (Thiers), 3-11cm, dark olive-brown to dark grayish olive, grayish brown, or brown, often tan to olive buff when old, (Arora), grayish brown to blackish brown (Trudell)
Flesh: "firm when young, becoming soft"; "whitish to pale yellow in pileus, yellow in stipe, bluing slowly to moderately or erratically", (Frank), 1-1.5cm thick, yellow, unchanging or becoming blue in irregular areas when exposed; in stem pallid to yellowish at top, "typically changing to reddish toward base, unchanging where exposed", (Thiers)
Pores: 1-3(4) per mm, "small and round at first, enlarging and becoming slightly angular", pale yellow to dingy greenish when old, "staining blue to bluish gray, slowly to moderately when bruised"; tube layer sunken around stem, 0.5-1.5cm long, (Frank), 1-1.5 mm, typically irregular, yellow, typically turning blue where bruised; tubes arched decurrent to depressed, similar in color, (Thiers)
Stem: 4-10cm x 0.8-2cm, "equal or with an enlarged base", "yellow to golden, streaked with fine red punctations with red pigment more concentrated toward base, often sparse or absent entirely towards apex", when old "becoming more red upward", (Frank), 7-10cm x 1-1.7cm, equal or narrowly slightly downward, solid; usually yellow at the top, becoming reddish toward base, "sometimes pallid toward base and reddish only in midportion, or entirely pallid with only the striations or ridges colored red"; dry, bald, often longitudinally ridged or striate, mycelium at base whitish to yellow, (Thiers), 4-13cm x 0.5-1.5cm, (Arora)
Odor: indistinct (Frank)
Taste: mild (Frank)
Microscopic: spores (12.5)12.9-16.2 x (4.7)5.1-6.2 microns, subfusoid to subcylindric, inequilateral to cylindric in side view, inamyloid; basidia 4-spored, 18.5-27.2 x 7.5-11.5 microns, clavate, colorless; hymenial cystidia infrequent, 38.9-49.5 x 11.6-11.9 microns, "ventricose to subclavate"; cap cuticle a trichoderm of cylindric to elongated cells, terminal cells 5-7 microns wide, "often narrower than those lower down", "pigment brown, parietal and incrusting, but light to absent in many terminal cells"; clamp connections absent, (Frank), spores 12-13.5 x 5-6 microns, elliptic to subventricose [wider in middle] to subcylindric [almost cylindric], smooth, not truncate, ochraceous in KOH and Melzer''s; basidia 4-spored, 33-36 x 7-9 microns, clavate, colorless; hymenial cystidia scattered to numerous, common only on pores, 56-75 x 10-13 microns, fusoid to fusoid-ventricose, colorless, thin-walled; cap cuticle a trichodermium with slightly differentiated hyphal tips, hyphae +/- 10 microns wide, often heavily and spirally incrusted; stem cuticle interwoven, incrusted; clamp connections absent, (Thiers)
Spore Deposit: dull olive brown (Frank), olive-brown (Thiers)
Habitat / Range
single "or scattered in troops", "under both conifers and hardwoods", "fall and early winter, or occasionally in spring on the California and Oregon coast, and summer in the Southwest, and at higher elevations", (Frank), single to gregarious under hardwoods and conifers, (Thiers)
Similar Species
Xerocomellus amylosporus has a darker cap "with more scattered and irregular cracks", a stem that develops brownish tones, "pores that bruise darker (inky) blue, a spore deposit that is more reddish brown in color, and larger, distinctly truncate spores", (Frank). X. amylosporus stains blue more quickly than X. diffractus, (N. Siegel, pers. comm.) |Xerocomellus rainisiae can be distinguished by its thick velvety cap cuticle, "the deep greenish black to bluish black staining" on the lower stem, and "the lack of extensive red pigmentation or red punctations" on the stem, (Frank). |Xerocomellus salicicola has a light brown to reddish cap "developing extensive cracking", an obscurely reticulate stem that is extensively red or retains a red band near the top, and grows with willow and birch, (Frank). |Xerocomellus mendocinensis "has quickly blue-staining pores" and a coarsely punctate stem "that is often extensively red or with a distinct red belt near the apex, and smaller truncate spores", (Frank). |Xerocomellus dryophilus has a more rosy red to pinkish brown cap, "stains blue more quickly and deeply", and grows with Quercus, but in dry conditions, fruitbodies can be hard to distinguish from X. diffractus, (Frank). |Xerocomus subtomentosus group is usually more olive to olive-brown on cap with no red in cracks, (Thiers). |Boletus ''spadiceus'' [here treated as a synonym of Boletus ''ferrugineus''] characteristically shows blue to blue-green discoloration when ammonium hydroxide is applied to the cap, (Thiers). |See also SIMILAR section of Hortiboletus rubellus.
