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Boletus illudens

[ Basidiomycetes > Boletales > Boletaceae > Boletus . . . ]

by Michael Kuo

Boletus illudens is an oak-loving species of Boletus from eastern North America, characterized by its relatively small size, the wide-meshed reticulum on the upper portion of its stem, the lack of any blue staining on its surfaces and flesh, and the blue-green reaction of its cap surface to ammonia. Its pore surface is yellow before it turns olive, and the pores themselves are large and angular.

Several medium-sized (rather than small) species are very close to Boletus illudens, including Boletus spadiceus, which also turns green with ammonia but, in theory anyway, has a slightly bluing pore surface--and Boletus subtomentosus, which does not turn green with ammonia. Boletus tenax (see the comments below) is so similar that it may be inseparable.

What Is Reticulation on Bolete Stems?

Boletus illudens, like many boletes, has a reticulate stem--and because the reticulation is so widely spaced and so clearly arranged at the apex of the stem, it is a good species for demonstrating that the tiny ridges comprising reticulation are in fact extensions of the pore surface. The criss-cross pattern formed by the ridges is like the pattern of the pores, but stretched out as a result of the stem's growth. The situation is roughly comparable, in gilled mushrooms, to gills that run down the stem ("decurrent" gills, in Mycologese). Microscopic examination confirms the idea that the reticulated ridges on the upper stem of Boletus illudens are actually "pores," since the ridges bear basidia--which are called "caulobasidia," since they occur on the stem--and spores (see the illustration).


Ecology: Mycorrhizal with oaks; growing scattered or gregariously; summer and fall; widely distributed in eastern North America.

Cap: 3-9 cm; convex, becoming broadly convex or nearly flat; dry; smooth or finely velvety; margin even; brownish to yellowish brown, reddish brown, or cinnamon.

Pore Surface: Yellow, becoming olive yellow or brownish yellow with maturity; not bruising; pores primarily angular, 1-2 mm wide; tubes to 16 mm deep.

Stem: 3-9 cm long; 0.5-1.5 cm thick; often tapered downward; dry; solid and tough; widely and coarsely reticulate near the apex (and often for half--or more--of the stem's length); whitish to pale brownish; basal mycelium whitish to yellowish.

Flesh: Pale yellow; not staining on exposure.

Odor and Taste: Not distinctive.

Chemical Reactions: Ammonia flashing blue-green on cap, then resolving to brownish with a greenish ring; negative on flesh. KOH dark brown on cap; negative on flesh. Iron salts bluish gray on cap; bluish gray on flesh.

Spore Print: Olive to olive brown.

Microscopic Features: Spores 10-14 x 4-5 µ; smooth; subfusiform. Pileipellis a trichoderm of tangled hyphae 4-12 µ wide, with scattered or frequent inflated and round elements 12-17 µ wide.

REFERENCES: Peck, 1897. (Saccardo, 1899; Coker & Beers, 1943; Singer, 1945; Snell & Dick, 1970; Smith & Thiers, 1971; Smith, Smith & Weber, 1981; Phillips, 1991/2005; Both, 1993; Bessette, Roody & Bessette, 2000; Kuo, 2007; Binion et al., 2008.) Herb. Kuo 07319702, 08300207, 08220501, 07050703.

Xerocomus illudens is a synonym.

Boletus tenax was described by Smith & Thiers (1971, p. 249) on the basis of a Michigan collection from Washtenaw County. "The outstanding features of this species," according to the authors, "are its poroid hymenophore, conspicuously reticulate stipe, the tenacious hold it has on the substratum, and the rusty brown incrustations on the cells toward the base of the trichodermial hyphae as revived in Melzer's." Like Boletus illudens, the cap of Boletus tenax turns green with ammonia. Macroscopic features are more or less inseparable between the two taxa--though Boletus tenax has a stubbier, more dramatically tapering stem. The pileipellis of Boletus tenax is similar to that of Boletus illudens, but it lacks the inflated elements that, according to Smith & Thiers, help define the taxon. Smith & Thiers's photo of Boletus tenax demonstrates the stubby, tapering stem; Roy Halling's photo of a collection in the New York Botanical Garden Herbarium displays the tapering stem and the cap's reaction to ammonia.

Further Online Information:

Boletus illudens in Smith & Thiers, 1971
Boletus illudens at Roger's Mushrooms


Boletus illudens

Boletus illudens

Boletus illudens

Boletus illudens

Boletus illudens

Boletus illudens

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Cite this page as:

Kuo, M. (2007, January). Boletus illudens. Retrieved from the MushroomExpert.Com Web site: