BOLETUS AEREUS Fries, Syst. Mycol. 1:393. 1821
Boletus edulis ssp. aerus (Fries) Konrad and Maublanc, Icon. Sel. Fung. 6:454. 1935.
Illustrations; See Microfiche no. 1
Kawamura, S., Icones of Japanese Fungi, pl. 241.
Leclair, A., and H. Essette, Les Bolets, pl. 35.
Romagnesi, H., Nouvel Atlas des Champignons, pl. 52.
Singer, R., Die Rohrlinge, Teil II, pl. V, figs. 1-6.

Pileus 10-15 cm broad when mature, convex to subglobose when young, becoming broadly convex to plano-convex to plane with age; surface moist, never viscid or tacky, often pitted or with broad depressions irregularly distributed, pits and depressions more numerous toward the margin, glabrous but with scattered white pruinose areas or patches; color dark brown to almost black ("warm sepia" to "bister" to "mummy brown" to "chestnut-brown"), with age sometimes irregularly fading on the disc to buff or reddish brown ("ochraceous-buff" to "cinnamon-rufous" to "Kaiser brown") with irregular areas almost white to tan ("light buff" to "pale olive-buff"); predominant color, however, much darker; margin slightly incurved and highly irregular when young, becoming decurved and even when older. Context 2-4 cm thick, vinaceous when young, with age white, except pale yellow above tubes and pale vinaceous below cuticle. Taste and odor mild.

Tubes 1.5-2.0 cm long, adnate to adnexed, becoming depressed; white when young, becoming yellow ("reed yellow") when mature; pores up to 1 mm broad, angular, concolorous, unchanging when bruised.

Stipe 8-11.5 cm long, 2.5-4 cm thick at the apex, clavate to subbulbous when young, unchanging or becoming more or less equal with age, solid; surface dry, reticulate at least over upper half, sometimes strongly and conspicuously so; color when young pallid ("pale pinkish buff" to "pale olive-buff") to almost white, unchanging or becoming vinaceous or vinaceous brown with age. Context white, unchanging when exposed.

Spore print dark olive brown. Spores 12-13.5 X 4-5 µm , pale ochraceous in KOH and Melzer's, subellipsoid to subfusoid in face view, inequilateral and ventricose in profile, walls thin, smooth.

Basidia 23-30 X 8-10 µm, hyaline in KOH, with ochraceous content in Melzer's, four-spored. Hymenial cystidia 28-34 X 7-11 µm , inconspicuous, embedded in the hymenium, hyaline in KOH and Melzer's, thin-walled.

Tube trama subgelatinous in KOH, hyaline to pale ochraceous, subdivergent, hyphae 4-5 µm wide. Pileus trama interwoven, homogeneous. Pileus cuticle a broad (175 µm ) trichodermium of erect, septate hyphal tips, terminal cells not enlarged, not incrusted, hyaline in KOH, pale ochraceous in Melzer's, hyphae 4-5 µm wide. Clamp connections absent.

Chemical reactions All tests negative.

Habit, habitat, and distribution Scattered to gregarious under oaks and madrones. It has been found only in the coastal forests of San Mateo County.

Material studied San Mateo County: Thiers 26918, 26926, 26948, 26963, 26978.

Observations The only other fungus known from the west coast with which B. aereus might be confused is B. edulis. Boletus aereus, however, has a very dark brown to blackish pileus that is moist to dry, but not viscid, shorter spores, and a different type of cuticle.

Singer previously recognized B. aereus from the eastern United States, but he did so by reducing such species as B. variipes and B. atkinsonii to synonymy with it. His action has not been generally accepted, and it appears certain he did not have California material at hand when making his decision. Careful comparison of the California material with European descriptions and collections show that the two are quite similar. Not only is the color of the pilei the same, but convincing similarities are also seen in the nonviscid cuticle (a feature emphasized by Watling), the spore size, and in the microscopic structure of the cuticle (a feature emphasized by Singer). In addition, the basidiocarps were collected under oaks on both continents.

There are two features in which the California material differs from that of Europe. First is the pruinose rather than granulose coating on the pileus and, second, the surface of the stipe and the reticulum are, in general, paler than that usually described for the European species. Neither of these characters seems to be of sufficient magnitude to prevent including them in this species concept.

Edible and delicious.

Online edition addendum

Other Descriptions and Photos: The Fungi of California

The Boletes of California
Copyright © 1975 by Dr. Harry D. Thiers
Additional content for the online edition © 1998 by Michael Wood, Fred Stevens, & Michael Boom
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