Lactarius crassiusculus
Lactarius crassiusculus H.T. Le & Stubbe
MycoBank number: 510722
Pileus 8-40 mm diam., convex when young, becoming plano-convex and depressed to almost infundibuliform, centre sometimes subumbonate, margin regular, somewhat wavy when older; surface dry and smooth to slightly rugulose, pale whitish, greyish cream (4B2/3) to pale brown (5BCD3/4), sometimes with olive brown tinges (4D3/4) in centre. Lamellae decurrent, close to crowded (12–16 L+l/1cm), 1-3 mm broad, 3 series of lamellulae, pale cream coloured to pale orange (3/4/5A2-3), when bruised staining at first greyish orange or dirty pinkish (6/7B5/6) but ultimately becoming brown (6DE5). Stipe 12-40 × 3-10 mm, more or less cylindrical, central or slightly eccentric; surface dry, smooth, greyish cream to dark cream coloured, staining brown (7C4–7E8) when bruised. Context moderately thick, 1.5-3 mm thick at mid-radius in pileus, solid or hollow in stipe, white, unchanging or because of the latex staining dirty pinkish, sometimes also yellowish, but later on becoming greyish brown; smell not remarkable or fragrant; taste mild or slightly acrid. Latex abundant, white, within a few minutes turning orange or pinkish (6A7–7B4–8B5), ultimately becoming dark coffee-and-milk coloured, grey brown or dark brown (8EF5). Spore print greyish orange (observed in LTH 281). Basidiospores (5.5)6.7–7.6–8.1–8.6 (9.5) × (5.3) 6.4–7.1–7.6–8.1 (8.8) μm (Q = 1.01–1.06–1.17; n= 100), globose to subglobose; ornamentation amyloid, forming an incomplete to almost complete reticulum with rather wide meshes, composed of firm ridges; ridges often with a slightly split aspect, < 2-2.5 μm high, mostly with smooth or somewhat irregular edges, sometimes slightly crenulate; minute isolated warts present between the ridges; plage mostly distally amyloid. Basidia 40-70 × 10-15 μm, subclavate to clavate, 4-spored, hyaline sometimes with guttate contents; sterigmata 3-6 × 1-2 μm. Pleuromacrocystidia abundant to very abundant, 40-80(95) × 8-13 μm, emergent, subcylindrical, subclavate or subfusiform, apex obtuse, acute, capitate or submoniliform, containing needle-like crystals, thin-walled. Pleuropseudocystidia abundant to moderately abundant, 2.5-5.5(6) μm diam., not or slightly emergent, slender, cylindrical, tortuous, thin-walled. Lamellar edge mostly sterile, sometimes basidia present but mainly composed of cheiloleptocystidia and cheilomacrocystidia; cheilomacrocystidia abundant, 40-60 × 9-11 μm, subclavate to fusiform, similar to pleuromacrocystidia, contents granular or with needle-like crystals, thin-walled; cheiloleptocystidia 10-25 × 2.5-8 μm, cylindrical to subfusiform or narrowly clavate, hyaline, thin-walled. Hymenophoral trama predominantly filamentous; lactifers abundant. Pileipellis a palisade, 60-90 μm thick, hyaline or containing a faint brownish intracellular pigmentation in the upper layers; suprapellis composed of subcylindrical, terminal elements that are 10-50 × 2-6.5 μm, sometimes septate, oblique to almost pericline, rather thin-walled but cell walls often somewhat refringent; subpellis composed of several layers of inflated, more or less isodiametric cells, 10-20 μm diam. Stipitipellis a trichoderm, 50-80 μm thick, sometimes inflated hyphal elements present, hyaline; hyphae 3-5(10) μm diam., thin-walled; terminal cells 18-35 × 4-8 μm, recumbent or erect, thin-walled.
Habitat and distribution: Growing on soil and leaves in mid- to highelevation broadleaf or mixed forests with Castanopsis, Lithocarpus and Pinus. So far only known from Northern Thailand.
Material Examined: THAILAND: Chiang Mai Province, Mae Taeng Distr., Tung Joaw village, N19°08.07’ E.098°38.09', 1423 m alt., rainforest dominated by Castanopsis armata, Lithocarpus sp. and Pinus kesiya, solitary to gregarious on the soil among leaves, 15/06/2005, Huyen T. Le 281 (holotypus: CMU; isotype: SFSU, GENT) – Chiang Mai Province, Doi Inthanon National Park, along Highway 1009 at 25 km marker, N18°32.56' E098°33.51', 1073 m alt., forest dominated by Castanopsis spp. and Pinus spp., 04/08/2005, leg. R. Zhao, Huyen T. Le 369 (CMU, SFSU, GENT) – ibid., 05/06/2006, leg. M.A. Neves, Huyen T. Le 398 (CMU, SFSU, GENT) – Chiang Mai Province, Mae Teng Distr., Highway 1095 at 22 km marker, N19°07.57' E98°45.65', 750 m alt., mixed forest dominated by Dipterocarpus spp. and Pinus kesiya, under Pinus sp., 02/07/2003, Huyen T. Le 12 (CMU, SFSU, GENT)– Chiang Mai Province, Doi Suthep-Pui National Park, Sangasahasri Lane to Huai Kok Ma village, N18°48.62' E098°54.60’, 1150 m alt., in primary montane forest with Castanopsis spp. and Lithocarpus polistachyus, 13/06/2006, leg. T. Osmundson, Huyen T. Le 439 (CMU, SFSU,GENT).
Notes: Lactarius crassiusculus is a pale, cream to pale brown coloured, medium-sized mushroom, with a moderately fleshy cap, dense lamellae, and latex turning pink and later on brownish. It is microscopically easily distinguished by the abundant macrocystidia, which are absent in the other Thai species. Although, macrocystidia are uncommon in L. subg. Plinthogali, the molecular analyses clearly confirm the position of L. crassiusculus within this subgenus. Morphologically, this classification is supported by the dull pileus colours, the pink discoloration of the latex, the globose spores with high (2-2.5 μm) reticulate ornamentation and the palisade in the pileipellis. Among the collected specimens certain morphological characters were not always consistent. In Huyen T. Le 369 a yellow discoloration of the context occurred after its turning pink, a character not observed in the other specimens. The ultimately brown discoloration of the latex was not as strong in every specimen. And Huyen T. Le 281 differed from the other specimens in having a hollow stipe. To eliminate any doubts concerning their conspecificity, all specimens were included in the molecular analyses. In all trees the specimens form a separate clade. We therefore interpret the observed morphological differences as part of intraspecific variability.
Fig. 1 MP phylogeny of Lactarius subgenus Plinthogali in Northern Thailand based on ITS sequences. The 50% majority rule consensus tree resulting from the Maximum Parsimony analysis with the bootstrap values higher than 50% added on the internal nodes.
Fig. 2 Lactarius crassiusculus. a. basidiomes. b. basidiospores, c. basidia, d. cheilomacrocystidia, e. cheiloleptocystidia, f. pleuromacrocystidia, g. pleuropseudocystidia. Scale bars = 10 mm (basidiomes) and 10 μm.
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