Cyanosporusflavus is characterised by flabelliform to semicircular and hirsute pileus with ash grey to light vinaceous grey pileal surface when fresh, buff to lemon-chrome pore surface when dry, and allantoid and slightly curved basidiospores (4.6–5.2 × 0.8–1.3 μm).

China. Sichuan Province, Jiuzhaigou County, on stump of Picea sp., 19.IX.2020, Cui 18547 (BJFC 035408).

Flavus (Lat.): referring to its lemon-chrome pore surface when dry.

Basidiomata annual, pileate, soft and watery, without odour or taste when fresh, becoming corky to fragile and light in weight upon drying. Pileus flabelliform to semicircular, projecting up to 3.2 cm, 5.7 cm wide and 0.9 cm thick at base. Pileal surface ash-grey to light vinaceous grey when fresh, becoming pale mouse-grey to mouse-grey when dry, hirsute; margin acute to slightly obtuse, white with a little blue tint when fresh, olivaceous buff to greyish brown when dry. Pore surface white to cream when fresh, becoming buff to lemon-chrome when dry; sterile margin narrow to almost lacking; pores angular, 5–7 per mm; dissepiments thin, entire to lacerate. Context white to cream, soft corky, up to 6 mm thick. Tubes pale mouse-grey to ash-grey, fragile, up to 4 mm long.

Hyphal system monomitic; generative hyphae with clamp connections, IKI–, CB–; hyphae unchanged in KOH.

Generative hyphae hyaline, thin- to slightly thick-walled with a wide lumen, occasionally branched, loosely interwoven, 2.7–6.5 μm in diam.

Generative hyphae hyaline, thin- to slightly thick-walled with a wide lumen, rarely branched, interwoven, 2.2–4.7 μm in diam. Cystidia absent; cystidioles present, fusoid, thin-walled, 12.3–17.8 × 2.2–3.5 μm. Basidia clavate, bearing four sterigmata and a basal clamp connection, 13.2–16.5 × 3.2–5.5 μm; basidioles dominant, in shape similar to basidia, but smaller, 12.6–15.7 × 2.9–5.2 μm.

Basidiospores slim allantoid, slightly curved, hyaline, thin- to slightly thick-walled, smooth, IKI–, CB–, 4.6–5.2 × 0.8–1.3 μm, L = 5 μm, W = 0.99 μm, Q = 4.96–5.25 (n = 60/2).

Brown rot.

China. Sichuan Province, Jiuzhaigou County, Jiuzhaigou Nature Reserve, on fallen trunk of Abies sp., 20.IX.2020, Cui 18562 (BJFC 035423).