North American Species of Lactarius, p. 282. 1979.
Common Name: none
Cap 3-7 cm broad, convex-depressed, plano-depressed in age; margin at first inrolled, then incurved, finally decurved; surface sticky when moist, more or less glabrous at the disc, elsewhere matted tomentose to fibrillose, the margin densely bearded; color: cream to cream-buff, tinged pale pinkish-orange, azonate to faintly zoned; context firm, white, unchanging, a narrow zone below the cuticle colored like the cap; odor mild, taste acrid.
Gills adnate to subdecurrent, crowded, cream, tinged pinkish-orange, often forked near the stipe, lamellulae up to four-seried; latex white, scanty, unchanging or yellowing slowly.
Stipe 2.5-4.0 cm long, 1.5-2.0 cm thick, dry, solid late into development, then hollow, brittle, equal, or the base narrowed to pinched; surface glabrous to inconspicuously pruinose (use hand lens), concolorous with the cap or lighter, typically not scrobiculate, sometimes darker where handled; partial veil absent.
Spores 6.5-7.5 x 4.5 x 5.0 µm, ellipsoid, with amyloid ornamentation consisting of raised warts and widely spaced ridges; spore print cream-yellow.
Scattered to gregarious under ornamental birches (Betula spp.); fruiting from late summer in watered areas, again after fall rains.
The above description has been written broadly to reflect the considerable variability reported for Lactarius pubescens var. betulae. Local material has a pale, azonate cap, white, unchanging latex, and a non-scrobiculate stipe, but some collections have been reported to have faintly zoned caps, a white, slowly yellowing latex, and may appear scrobiculate. The association with ornamental birches is an important field character and helps to distinguish this species from Lactarius torminosus var. nordmanensis. The latter according to Methven, occurs only in montane areas under Salix and Populus species, and has larger spores.
Methven, A.S. (1997). The Agaricales (Gilled Fungi) of California. 10. Russulaceae II. Lactarius. Mad River Press: Eureka, CA. 79 p.
Hesler, L.R. & Smith, A.H. (1960). Studies on Lactarius II--The North American Species of Sections Scrobiculus, Crocei, Theiogali, and Vellus. Brittonia 12(4): 305-350.
Hesler, L.R. & Smith, A.H. (1979). North American Species of Lactarius. University of Michigan Press: Ann Arbor, MI. 841 p.