Armillaria desertorum T. Bau 2025, sp. nov.

Armillaria desertorum T. Bau, sp. nov. Figs 2, 3 MycoBank: —MB857371 Diagnosis: —Pileus white to reddish brown, scales sparse yellow-brown scaly, with veil remnants, annulus superior and white. Stipe base with black thick rhizomorph. Basidiospores ellipsoid, smooth, inamyloid. Basidia with 4 (2) sterigmata. Etymology: — desertorum represents the desert habitat from where the type specimen was collected for the first time. Type: — CHINA. Inner Mongolia: Ordos city, Hanggin Banner, Jirgalant Town, Gegengzhao Gacha, 25 Oct. 2024, coll. T. Bau, 39°27'19"N, 110°2'11"E, alt. 1222 m, AHE2(FJAU 69322, holotype!). Description: — Pileus 0.5–6.2 cm, hemispherical to convex when young, becoming applanate after maturation, the center slightly concave, fleshy. The surface of pileus milky white (RAL 9010) to dark ivory (RAL 1019) when young, light reddish-brown (RAL 3012), with heather violet (RAL 4003) center when mature, smooth or with sparse yellow-brown (RAL 8023) scaly scales, often covered with sand. The edge of pileus curved inwards, prone to falling off, hygrophanous, longitudinal striations and appendiculate, then easily cracked radially. Context white (RAL 9003), with a sweet, greasy odor. Lamellae adnate to decurrent, milky white (RAL 9010), slightly dense (L = 24–32, I =1- 2) to dense (L = 42–48, I = 1-2), 2–6 mm wide, unequal length, the edges intact. Stipe 2.7–7.2 cm long, 0.3–1.5 cm wide, cylindrical, slender, creamy (RAL 9001) to beige (RAL 1001), fleshy, solid, the surface prone to cracking when dry. Annulus superior, white (RAL 9003), thick, the edge initially curving upwards, then flattening and finally curling downwards, membranous. Rhizomorph dark (RAL 9004) and robust. Basidiospores {40/3/3} (7.6) 8.5–9.1 (10.7) × (6) 6.9–7.5 (9.2) µm, Q =1.18–1.25 [1.22±0.09, 1.23], ellipsoid, transparent, colorless, smooth, thick-walled, smooth, inamyloid. Basidia 34–48×7–11μm, long clavate, with 4 (2) sterigmata, sterigmata 3–6.7μm long, slightly thick-walled to thick-walled. The trama of lamellae bilateral type, hyphae 3.3–4.7 μm wide, thin-walled and colorless. The terminal elements of lamellae a. fusiform to gourd-shaped, 9.6–32×8.1–13 μm, septate, hyphae connected at the base with short branches occasionally; b. clavate, sometimes slightly constricted in the middle, 30–44×6.7–8.9 μm, thick-walled, septate, the part above septum 6.8–18×7.3–9.4 μm. Pileipellis cutis-type, hyphae 6–22 μm wide, the terminal cells nearly cylindrical, sometimes with blunt fingerlike projections at the top, thin to slightly thick walls, light brown to hyaline. Hyphae on the surface of annulus 4–75× 6–14 μm, interwoven with each other, the terminal cells 17–28×6–13 μm, ellipsoid or nearly clavate, sometimes filamentous, and irregularly curved. Caulocystidia absent. Clamp connections absent in all tissues. Habitat: —Buried or semi-buried in sandy land, near the roots of rotten Caragana korshinskii and Populus euphratica. Distribution: —Currently only known from Inner Mongolia. Additional specimens examined: — CHINA. Inner Mongolia: Ordos, 40°50’– 40°51’N, 107°57’– 107°58’E, alt. 1000 m, 6 oct. 2024, coll. T. Bau (FJAU69321, paratype!); Inner Mongolia: Alxa League, Ejin Banner, Dalaihob Town, ulanggerile Gacha, 25 Nov. 2024, coll. Sukh, 37°24’– 42°02’N, 97°10’– 101°18’E, alt. 1000–1222 m, AHM3 (FJAU69323).