Pileus: 2.0 - 10.5 cm broad; umbilicate to convex at first, then planar and depressed and finally flaring to funnel shaped; margins inrolled, becoming straight and even when mature, wooly especially when young, becoming more fringed then glabrous at maturity; surface moist to viscid, with reddish brown fibrils radiating from disc, then glabrous, at times pitted, at times zonate; deep yellow on disc fading to yellowish white on margins when young, becoming pale yellow to reddish yellow when mature; context 7 mm at junction with stipe, firm, fleshy, white to ivory, unchanging; taste bitter at first then slowly becoming peppery; odour distinctive, variously described as fruity, mushroom like.
Latex: White at first, quickly turning yellow, bitter taste.
Lamellae: Horizontal at first then subdecurrent finally decurrent; distantly spaced, forking towards stipe, 4 mm broad, edges even, fragile, cracking easily, yellowish white to cream when young, becoming light yellow when mature, bruising yellow brown; 1 lamellula.
Stipe: 1 - 6 cm long X 0.5 - 2.0 cm wide; even and straight, surface dry, scrobiculate especially at base, at times glabrous, when young pallid to reddish yellow to deep yellow, becoming paler when mature, pits and where handled becoming brownish yellow to light brown; context moist, solid at first, becoming hollow when mature, rind thick, up to 1 cm, colour pallid.
Comments: This is the only yellow Lactarius found in these forests. It is easily recognized by the wooly cap margins, pitted stipe, the white latex that quickly turns yellow, and the bitter to peppery taste. Methven (1997) separates L. scrobiculatus into 3 varieties based on colour, microscopic features, and habit. L. scrobiculatus var. pubescens has a pale yellow pileus and smaller spores than L. scrobiculatus var. montanus. L scrobiculatus var canadensis, has cheilocystidia and densely bearded margins.
Hesler, L.R. and A.H. Smith. 1979. North American species of Lactarius. Univ. Michigan Press. Ann Arbor, Mich. pp 841.
Methven, A.S. 1997. The Agaricales (gilled fungi) of California. 10. Russulaceae II. Lactarius. Mad River Press. Eureka Ca. pp 78.