Pileus 5-10 cm broad when mature, broadly conic to convex when young, becoming broadly convex to plane to umbonate at maturity; surface viscid, when very young appearing glabrous to streaked with gluten, sometimes with plaques or scales toward the margin, with age glabrous on the disc or with scattered inconspicuous plaques to conspicuous plaques on the disc; when young background color yellow ("mustard yellow" to "Isabella color") with the gluten and plaques colored reddish ("russet"), with age background changing to dark yellow ("chamois" to "honey yellow") and the plaques brown to reddish brown ("Mikado brown" to "pecan brown"), tending to stain vinaceous when bruised; margin strongly incurved to inrolled, when young with a highly conspicuous roll of cottony veil tissue colored whitish to pale yellow, but with age whitish to grayish white, becoming glabrous with age. Context 1-2 cm thick, yellow ("Naples yellow"), but typically staining vinaceous when exposed. Taste of gluten acid; otherwise taste and odor mild.
Tubes 7-10 mm long, decurrent, when very young yellow ("mustard yellow"), darkening to dull yellow ("antimony yellow" to "yellow ocher") with age, staining vinaceous to dirty pink when bruised; pores up to 2 mm broad, elongated to rectangular, typically radially arranged but not always so, compound, concolorous with tubes.
Stipe 5-11 cm long, 0.5-1 cm thick at the apex, equal, solid, sometimes appearing hollow with age; surface moist to subviscid, staining fingers black when handled, glandular, glands pallid and inconspicuous when young, becoming brown to yellow brown with age; background color yellow ("Naples yellow" to "mustard yellow"), staining brown to vinaceous when bruised; annulus sometimes present, often evanescent.
Spore print brown. Spores 8-12 X 3.8-4.5 Ám, subellipsoid to subcylindric, ventricose in side view, smooth, thin-walled.
Basidia 22-34 X 5-8 Ám, clavate, four-spored. Hymenial cystidia 50-75 X 6-9 Ám, clustered, hyaline but staining dark brown in KOH, scattered to abundant, subclavate to cylindric.
Tube trama divergent, gelatinous in KOH, hyaline. Pileus trama interwoven, homogeneous. Pileus cuticle differentiated as an ixotrichodermium, hyphae 3-6 Ám wide. Caulocystidia similar to the cystidia in the hymenium. Clamp connections absent.
Chemical reactions KOH-context flashing red then blackening; FeSO4 -slowly grayish on cuticle and context.
Habit, habitat, and distribution Scattered to gregarious in soil under western white pine. This species has not yet been recorded from California so far as is known. It is common, however, in several western states, and is included here since western white pine is relatively widely distributed within the northern half of the state.
Observations Earlier reports by Smith et al. have pointed out the apparent confusion between this species, S. umbonatus, and S. americanus. There are many close similarities between S. sibiricus and S. umbonatus and the two are frequently misdetermined. Suillus umbonatus generally has larger pores, a somewhat smaller, more umbonate pileus that is often pallid in color, and is associated with lodgepole rather than five-needle pine. It is readily apparent that S. americanus is very close to S. sibiricus, and considerable difficulty is encountered in separating these two species. Perhaps the most obvious and constant difference is seen in the tendency of the context of the stipe base to stain vinaceous in S. sibiricus. As has already been indicated, it is now believed that S. americanus does not occur west of the Rocky Mountains and that many reports of its presence in the west have been based upon basidiocarps of S. sibiricus.
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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