Psathyloma leucocarpum Soop, J.A. Cooper & Dima, in Soop et al., Mycologia 108(2): 400 (2016)
Etymology: from leukos (Greek) “white” and “karpós” (Greek) referring to the general appearance of the basidiomata.
Pileus 20–70 mm diam, rounded-conical, later expanded to almost plane, viscid, hygrophanous, white to gray-white (2B1) with a grayish yellow (2B3, 2B2) disk, glabrous; margin without veil remnants, not striate. Lamellae pale gray when young, later brownish (6C3, 7C3), fairly crowded, adnate to narrowly emarginate; edge slightly fimbriate. Stipe 35–65(–110) x 4–10 mm, cylindrical, dry; silvery white to graywhite, slightly zoned without visible veil remnants. Veil absent or fugacious; cortina absent. Context white. Odor insignificant; flavor faintly agaricoid or sweetish. Macrochemical reactions: NaOH negative or weakly yellow in context. Spore print cinnamon (6D5).
Basidiospores (6.5–) 6.6–7.6 (–7.7) x (3.9–) 4.0–4.4 μm. Q = 1.6–1.9; mean 7.1 x 4.2 μm, Q = 1.7; N = 20, ellipsoid in face view, amygdaliform in side view, with lateral apiculus, smooth, pale brown, thick-walled, dextrinoid, often with refractive content in Melzer’s, with a broad germ pore to 1.0 μm diam. Basidia 20–30 x 5–7 μm (excluding sterigma), clavate to subcylindrical, with basal clamp, four-spored, sterigma to 6 mm long x 2 mm wide at base. Pleurocystidia not observed. Cheilocystidia abundant (lamellar edge sterile), variable shape (clavate, lageniform, sphaeropedunculate), thinwalled, hyaline, with basal clamp, 25–40 x 3–10 μm. Pileipellis an ixocutis up to 70 μm thick composed of cylindrical, smooth, thin-thick-walled, hyaline hyphae to 5 mm diam embedded in a gelatinized matrix. Hypocutis of broadly globose, hyaline cells up to 20 μm diam over a dense layer of parallel hyaline hyphae. Pilea and lamella tissue inamyloid not dextrinoid. Stipitipellis a closely packed cutis of glassy-walled hyphae 5–10 μm diam. Caulocystidia not observed. Clamp connections present in all tissues.
Habitat: Gregarious, common, associated with Nothofagaceae spp.
Index fungorum number: IF812052
Notes: Psathyloma leucocarpum is somewhat larger than Ps. catervatim; this species differs from the latter mainly by being considerably paler, sometimes entirely white. Microscopically the two species are nearly identical. Ps. leucocarpum resembles the common species Psathyrella candolleana (Fr.) Maire, except that the remnants of the veil at the margin of the pileus are missing and the pileus is moist to viscid. Psathyloma leucocarpum forms a separate clade, well distinguished from the Ps. catervatim clade.
Figure 1. Majority-rule consensus phylogram of the Hymenogastraceae inferred with MrBayes from the concatenated alignment of ITS and 28S sequences. Numbers above or below branches represent nodal supports (posterior probability/ML bootstrap percentage); values above 0.5 and 50% are shown. Voucher numbers are indicated only at newly generated sequences except those of two Hymenogastraceae sequences retrieved from GenBank. New Psathyloma sequences marked in boldface. Bar indicates 0.01 expected change per site per branch.
Figure 2. Maximum-likelihood phylogram of Psathyloma and related environmental ITS sequences. Numbers above or below the branches represent nodal support (bootstrap percentage), values above 70% only are shown.