Pluteus ursulus E.F. Malysheva, 2025

Pluteus ursulus E.F. Malysheva , sp. nov. ( Figs. 2 View FIGURE 2 , 3 View FIGURE 3 )

MycoBank: MB 856874

Diagnosis:—Basidiomata small to very small, pileus minutely squamulose with dark brown squamules and cracked surface, lamellae edges pigmented; basidiospores (4.5)5.3‒6.8(8.2) × 4.2‒5.5(6.8) µm, highly variable in size, mostly ellipsoid to broadly ellipsoid, pleurocystidia fusiform or utriform, with brown intracellular pigment, cheilocystidia clavate or lanceolate, with brown content, clamps absent; growing on rotten wood of deciduous trees.

Holotype:— RUSSIA. Primorye Territory, Sikhote-Alin State Nature Biosphere Reserve, vicinity of Khanov field reserve station, 44.896943°N, 136.337051°E, floodplain forest ( Betula platyphylla, Populus tremula, Ulmus japonica , Quercus mongolica ) at Khanov brook, on small twigs of Ulmus japonica in the litter, 17 Aug. 2024, V. Malysheva (LE F-354560). GenBank ITS: PQ699264; TEF 1: PQ723748.

Etymology:—The specific epithet “ursulus ” (young bear) emphasizes the association with the image of a bear cub, which is smaller than an adult bear, due to the small size of basidiomata, brown color of pilei and their velvety pubescence.

Description:— Pileus 5‒15 mm in diam., plano-convex or applanate with small broad umbo or concave centre; not hygrophanous, not striate, with uneven margin; madeira brown (RAL 050 40 40), rosewood brown (RAL 050 30 36) or tobacco brown (RAL 050 30 30), with darker night brown (RAL 050 20 16) centre; surface dry, densely covered with small protruding scales, radially arranged on the pileus surface and dense in the centre, forming a contrasting central disc; pileus surface markedly cracked and white background is visible on the margin between the scales. Lamellae free, rather distant, slightly ventricose, pinkish, with uneven, slightly crenulated brown edges. Stipe 8‒20 × 1‒1.5 mm, slightly broadened towards base, with small subtle bulb, solid, whitish, all covered with light milk coffee brown (RAL 050 50 20) or dark tobacco brown (RAL 050 30 30) dotted squamules. Smell absent, taste not recorded.

Basidiospores (4.5)5.3‒6.8(8.2) × 4.2‒5.5(6.8) µm (Lav = 6.0, Wav = 4.9, Q = (1.0)1.1‒1.3(1.6); Qav = 1.2), highly variable in size, ellipsoid to broadly ellipsoid, some ovoid or subglobose, thick-walled. Basidia 15‒20 × 6.2‒8.0 µm, 4-spored, sporadically 2-spored, clavate. Pleurocystidia 44.5‒56.5(62.7) × 15.5‒29.0(36.2) µm, sporadic, rare, broadly fusiform, utriform, rarely lageniform, usually pedunculated, with brown intracellular pigment, slightly thick-walled. Cheilocystidia (21.2)31.0‒50.0(60.5) × 12.0‒29.0(36.5) µm, abundant, forming a sterile layer at the edge of lamellae, mostly broadly clavate, sometimes lanceolate with a pointed apex, rarely utriform, with intracellular brown pigment, thin-walled. Pileipellis a hymeniderm, composed of two types of elements: 1) spheropedunculate, pyriform, broadly clavate elements, (28.5)33.5‒64.5 × (17.0)23.0‒35.7(38.2) µm; and 2) long cystidioid elements, broadly to narrowly fusiform with acute apices, (62.0)80.0‒146.5(176.0) × (13.3)16.5‒28.0(37.0) μm; all elements with brown intracellular pigment unevenly distributed in spots in the cell, thin- or slightly thick-walled. Stipitipellis a cutis of cylindrical, slightly thick-walled, 9‒12 µm wide hyaline hyphae. Caulocystidia numerous, in bundles, (31.3)52.3– 70.8(92.0) × 8.5–15.5(17.5) μm, narrowly fusiform to lageniform or narrowly clavate, some with long narrow neck, often with sinuous and acute apex, single caulocystidia septated, slightly thick-walled with brown intracellular contents. Clamp-connections absent in all tissues.

Habitat and distribution:—In small groups, on hardwoods. Currently, only known in the Primorye Territory, Russia.

Additional collection examined:— RUSSIA. Primorye Territory, Sikhote-Alin State Nature Biosphere Reserve, vicinity of Khanov field reserve station, 44.895000°N, 136.334333°E, floodplain forest ( Betula platyphylla, Populus tremula, Ulmus japonica , Quercus mongolica ) at Khanov brook, on a fallen moss-covered hardwood tree, 17 Aug. 2024, V. Malysheva (LE F-354562). GenBank ITS: PQ699265; TEF 1: PQ723749.