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Boletus hortonii

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

by Michael Kuo

This fascinating bolete is relatively common in eastern North America's oak forests. It has a tightly wrinkled cap surface that turns green with ammonia, a yellow pore surface, and a stem that appears bald and unadorned but on close inspection features tiny scabers. The flesh and pore surface occasionally bruise slightly bluish, especially in older specimens.

Mycologists have long struggled with how to fit Boletus hortonii into taxonomic arrangements. It was originally named Boletus subglabripes var. corrugis by Peck (1889, p. 112), since it was so similar to Boletus subglabripes (now Leccinum subglabripes), appearing to differ only in its consistently wrinkled cap. Smith & Thiers (1971) elevated Peck's variety to species status, renaming it in his honor (Peck's middle name was Horton) in the process. Boletus hortonii is indeed very similar to Leccinum subglabripes, but it is occasionally stocky and not very Leccinum-like. Additionally, the structure of its pileipellis is different, and the green reaction to ammonia is not found in Leccinum subglabripes or anywhere else in Leccinum. DNA results (den Bakker & Noordeloos, 2005) have tentatively confirmed placement of Leccinum subglabripes in Leccinum, but Boletus hortonii has yet to be reliably tested, to my knowledge.


Ecology: Mycorrhizal with oaks and perhaps with other hardwoods; growing alone, scattered, or gregariously (sometimes densely so); early summer through fall; northeastern United States and eastern Canada.

Cap: 3-14 cm; convex, becoming broadly convex; dry or greasy; conspicuously, tightly wrinkled; bald; reddish brown to cinnamon or medium brown, but often fading in age.

Pore Surface: Yellow, becoming dull yellow or olive; not typically bruising, but sometimes bruising slowly cinnamon or bluish; with 2-3 pores per mm; tubes to about 1 cm deep.

Stem: 6-10 cm long; 1-2 cm thick; more or less equal, or enlarging slightly to base; dry; yellow at the apex; elsewhere pale yellow to tan, sometimes flushed reddish; adorned with minute scabers that are initially yellow and concolorous with the surface but often turn reddish with age or handling; basal mycelium whitish.

Flesh: White to yellowish; not typically staining on exposure, but sometimes bluing slowly and weakly.

Odor and Taste: Not distinctive.

Chemical Reactions: Ammonia green (or flashing green and then resolving to brownish or gray) on cap surface; negative on flesh. KOH blackish to brownish on cap surface; negative to orangish on flesh. Iron salts negative to pale olive on cap surface; negative to bluish gray on flesh.

Spore Print: Olive brown.

Microscopic Features: Spores 12-15 x 3.5-4.5 µ; smooth; subfusiform. Pileipellis epithelium-like, but with frequently tubular (rather than inflated) terminal elements and inflated sub-terminal elements.

REFERENCES: Smith & Thiers, 1971. (Peck, 1889; Saccardo, 1891; Smith & Thiers, 1971; Smith, 1973; Smith, Smith & Weber, 1981; Phillips, 1991/2005; Both, 1993; Bessette, Roody & Bessette, 2000; Roody, 2003; McNeil, 2006; Kuo, 2007; Binion et al., 2008.) Herb. Kuo 05300407, 07050706, 06180804.

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Boletus hortonii

Boletus hortonii

Boletus hortonii

Boletus hortonii

Boletus hortonii

Boletus hortonii

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Kuo, M. (2010, April). Boletus hortonii. Retrieved from the MushroomExpert.Com Web site: