CHINA. Hainan Province, Wuzhishan County, Wuzhishan Nature Reserve, on fallen angiosperm branch, 10 Jun 2016, He 3968 (holotype, BJFC 022470).

“tropicum” refers to the distribution in tropical areas.

Annual, resupinate, effused, very thin, separable from the substrate, up to 10 cm long, 3 cm wide. Hymenophore surface smooth, white (5A1), orange grey (5B2) to greyish-orange [5B(3–4)], not cracked; margin undifferentiated.

Hyphal system monomitic; generative hyphae with clamp connections, hyaline, slightly thick-walled, frequently branched and septate, loosely interwoven, 2–3.5 µm in diam. Cystidia abundant, subulate, projecting beyond hymenium, hyaline, thick-walled and regularly covered with rectangular crystals except at the apex, 40–70 × 3–5 µm. Basidia subclavate to suburniform, hyaline, thin-walled, with 4 sterigmata and a basal clamp connection, 12–17 × 4–5 µm; basidioles in shape similar to basidia, but slightly smaller. Basidiospores fusiform to slightly vermicular, hyaline, thin-walled, smooth, negative in Melzer’s reagent, acyanophilous, 11–12.5(–13) × 1.8–2.2 µm, L = 11.9 µm, W = 2 µm, Q = 5.95 (n = 30/1).

CHINA. Hainan Province, Baoting County, Qixianling Forest Park, on fallen angiosperm branch, 18 Mar 2016, He 3583 (BJFC 022083).

Subulicystidiumtropicum resembles S.acerosum and S.perlongisporum Boidin & Gilles by sharing narrow basidiospores in the genus, but differs from S.acerosum in having shorter basidiospores and lacking the needle-like crystals and from S.perlongisporum in having much shorter basidiospores and a tropical distribution (16–25 × 1.5–2.5 µm for S.perlongisporum in Ordynets et al. 2018). The new species is also similar to S.longisporum, but differs in having slender basidiospores and a tropical distribution. In the phylogenetic tree, S.tropicum formed a distinct lineage in Subulicystidium (Fig. 2).