The detailed description of the species provided below is from Mykoweb Boletes of California section: and was originally derived from Thiers 1975^1^ book .
Pileus 4-14 cm broad when expanded, obtusely convex to convex when young, becoming broadly convex to plano-convex with age; surface viscid to glutinous during all stages of development, glabrous but sometimes appearing obscurely streaked from gluten when older; color when young white to olive (“deep grayish olive” to “grayish olive” to “citrine drab”) with splotches or irregularly shaped areas colored pale olive (“pale olive-buff” to “olive-buff”), frequently strongly variegated with a mixture of light and dark colors, when older unchanging or becoming reddish brown (“ochraceous-tawny”), sometimes a mixture of all the pigments mentioned above; margin incurved and with a cottony roll of white tissue when young, becoming naked and merely decurved with age. Context 1-2 cm thick, white and unchanging in young basidiocarps, frequently changing to yellow (“pinard yellow”) when older. Taste harsh, subnauseous, and weakly acid; odor strong, pungent.
Tubes up to 1 cm in length, adnate when young, becoming decurrent to subdecurrent with age; color when young whitish to pale buff (“cartridge buff”) with conspicuous whitish droplets that become brown to ochraceous when dried, with age changing to near yellow (“colonial buff”) and finally to dark yellow (“honey yellow”); pores 1-1.5 mm broad, not radially arranged, unchanging when bruised, angular.
Stipe 3-7 cm long, 1-2 cm thick at the apex, equal to tapering at the base to sometimes subventricose, solid; surface dry, glabrous, strongly punctate, glandulae large, irregular in outline, reddish at first then becoming brownish, background whitish to more or less concolorous with the tubes when young, becoming yellow (“pinard yellow” to “massicot yellow”) with age, unchanging when bruised; no annulus. Context white, unchanging when exposed.
Spore print brown. Spores 9.5-10 X 2.8-3.5 µm, hyaline in KOH, smooth, thin-walled, ellipsoid to subcylindric in face view, inequilateral in profile.
Basidia 33-36 X 8-10 µm, hyaline, clavate, contents granulose in KOH, four-spored. Hymenial cystidia 43-79 X 7-10 µm, rare to scattered, abundant on the pores, typically occurring in massive clusters, dark brown in KOH, cylindric to subclavate, incrusted, occasionally hyaline, thin-walled.
Tube trama hyaline, divergent to subparallel, hyphae 3-5 µm wide. Pileus trama interwoven, homogeneous. Pileus cuticle differentiated as an ixotrichodermium, staining brown in KOH, hyphae 4-5 µm wide. Stipe cuticle with clusters of cystidia similar to those found in the hymenium. Clamp connections absent.
Chemical reactions KOH-context vinaceous, tubes red, pileus cuticle black, stipe cuticle pale vinaceous; NH4OH-context very pale vinaceous, tubes bright red; FeSO4-context gray, tubes dark gray to black, stipe cuticle light gray.
Habit, habitat, and distribution Solitary to gregarious in humus under Monterey pine. This is often the most abundant Suillus in the San Francisco Bay Area, and although it is usually found under Monterey pine, a few collections have been made under knobcone pine. The type collection was made on the campus of San Francisco State University in San Francisco, where it occurs in abundance during the fall and winter seasons. It is one of the few species of Suillus that continues to fruit sporadically throughout the year.
Observations This species is distinctively characterized by the very noticeable color changes occurring in the pileus as it develops. In most instances the young pilei are pure white, as development continues they next become dark gray, and finally at maturity they typically change to rust brown in color. The tubes, when young, are almost white but become yellow with age. If the humidity is high, there is usually an abundance of whitish droplets on the hymenophore, at least during the young stages. The ring of cottony veil tissue on the margin of the pileus is very pronounced during the younger stages but, as is often the case in the genus Suillus, it disappears with age. Suillus pungens has been misidentified as S. placidus because of the white color of the young basidiocarps and the droplets of exudate. The pallid to white colors, the cottony roll of veil tissue, and the unpleasant taste and odor distinguish it from S. granulatus and S. albidipes.
Edible, but not choice.