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Boletus badius

[ Basidiomycota > Boletales > Boletaceae > Boletus . . . ]

by Michael Kuo

Although several of its defining features can vary between collections, Boletus badius is fairly easily recognized. It is found in the conifer forests of northeastern North America, and occasionally in northern beech-maple or birch forests. It is often found fruiting from very decayed wood, and the "bay brown" color of its cap and stem contrast elegantly with the pale yellow pore surface, which bruises bluish or grayish.

The surface of the stem is distinctive, in my opinion, and is rarely described sufficiently--perhaps because it is a hard thing to describe. While the stem is not reticulate, it is also not smooth; broad, shallow wrinkles cover at least the midportion when the mushroom is young.

Boletus badius has often been placed in the genus Xerocomus, especially by European authors. But the most recent name for the fungus is "Imleria badia," by virtue of an unscientific, brief online publication at Index Fungorum (Vizzini, 2014).


Ecology: Mycorrhizal; growing alone or scattered, on the ground or from well decayed wood; under eastern hemlock and other conifers, or occasionally in beech-maple or birch woods; summer through fall; northeastern North America and the northern Midwest. The illustrated and described collection is from Michigan.

Cap: 3-9 cm; convex, becoming broadly convex; sticky when fresh, but soon dry; finely velvety or bald; reddish brown to brownish red; margin often with a tiny extending sterile portion.

Pore Surface: Pale yellow when young, becoming greenish yellow to olive; bruising promptly grayish to bluish or greenish blue; 1-3 pores per mm; tubes to 1 cm deep.

Stem: 5-15 cm long; 1-3 cm thick; more or less equal, or club-shaped when young; solid; yellowish to whitish at the apex; colored like the cap or slightly paler below; often featuring broad, shallow wrinkles when young; basal mycelium white; without reticulation.

Flesh: Whitish, often staining yellowish on exposure, and weakly bluish over the tubes--but sometimes not staining.

Odor and Taste: Not distinctive.

Chemical Reactions: Ammonia dark red to black (sometimes with an olive ring of color, or first flashing green) on cap surface; negative to brownish on flesh. KOH blackish maroon on cap surface; pale orangish on flesh; golden brown on pore surface. Iron salts dull bluish green on cap surface; pale yellow to olive on flesh.

Spore Print: Olive.

Microscopic Features: Spores 9-14 x 3-4 µ; subfusiform; smooth; yellowish in KOH; often appearing biguttulate and/or thick-walled. Hymenial cystidia fusoid-ventricose to mucronate, 30-60 x 9-15 µ. Pileipellis a trichoderm; elements 5-7.5 µ wide, smooth, hyaline to yellowish in KOH.

REFERENCES: (Fries, 1818) Fries, 1821. (Saccardo, 1888; Coker & Beers, 1943; Snell & Dick, 1970; Smith & Thiers, 1971; Grund & Harrison, 1976; Smith, Smith & Weber, 1981; Phillips, 1991/2005; Lincoff, 1992; Both, 1993; Barron, 1999; Bessette, Roody & Bessette, 2000; Roody, 2003; McNeil, 2006; Miller & Miller, 2006; Kuo & Methven, 2014.) Herb. Kuo 09110404.

A "collective species?"

Smith and Thiers (1971) warn that "this species is collective and deserves further study" in North America, and that "we need to sharpen our concept of B. badius" (308, 310-11). Just flip through the major North American field guides to compare photos and descriptions, and you will see what they mean. One issue has been the blue staining (or lack of it). In 1887, Peck wrote that he "observed no greenish hue to the tubes nor bluish color to the flesh" (see Both, 41), in contrast to European descriptions of the species, which called for blue staining. Yet collections identified by later North American mycologists sometimes stained weakly blue in the flesh, especially over the tubes. With the current species concept, it seems to me, the best way to describe the staining is to say that it is variable--from completely absent to weakly blue over the tubes. The bruising of the pore surface is also variable, ranging from grayish to bluish or bluish green.

Spore size is another point of contention; Smith and Thiers document collections with spores as short as 9 µ and as long as 18.5 µ. Grund and Harrison (1976) record a variant, Boletus badius var. macrostipitatus, with spores 15-18 µ long (and a shorter, stouter stem). The cystidia have also varied between mycologists' collections.

As far as the chemical reactions are concerned, a giant mess has evolved from inexact wording on the part of Smith and Thiers, and errant interpretation of this wording by Bessette, Roody & Bessette (2000). Though it is likely that no one cares, I will attempt to sort out this mess here so that future Boletus badius fans do not have to wonder. Here is what Smith and Thiers write:

    Context whitish young . . . rarely bluish when injured; odor slight, taste sour to mild, with FeSO4 dull bluish green, with KOH on tubes golden brown; NH4OH olive around the spot of application of the drop.

Mycological giants though they were, Smith and Thiers were clearly better at identifying mushrooms than using semicolons and commas; the arrangement of punctuation in the passage makes no sense and is misleading. What we don't know, reading this passage, is what surface of the mushroom is dull bluish green with iron salts (FeSO4), or what surface is olive with ammonia (NH4OH). Because the sentence begins with the word "context," perhaps, Bessette and his co-authors interpret the iron salts reaction to apply to the flesh--yet they interpret the ammonia reaction to apply to the cap surface.

I have tested the chemical reactions of Boletus badius for myself, and the color changes (used in the description above) make it clear that Smith and Thiers probably meant that the cap surface, not the flesh, is dull bluish green with iron salts (contrary to the interpretation of Bessette, Roody & Bessette). The ammonia reaction also apparently applies to the cap surface.

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Boletus badius

Boletus badius

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Kuo, M. (2014, December). Boletus badius. Retrieved from the MushroomExpert.Com Web site: