THAILAND, Ubon Ratchathani Province, Trakan Phuet Phon District, Don Khok Tam Lae community forest, 15°35'46"N, 105°06'38"E, elev. 150 m., 28 July 2015, S. Vadthanarat 170, (holotype: CMUB!; isotype: BR!).

From Latin “rostrati–” meaning having beaked prow or a solid projection and “spora” meaning spores, referring to the basidiospores having a markedly prominent and large hilar appendage.

Basidiomata small to medium-sized 0.8–2.5(4.5) cm diam., subglobose to ovoid with a rudimentary elongated basal attachment, with greyish white to pale brown rhizoids at the base and going up along the surface of basidiomata to about half of the height. Peridium surface (outer peridium) fibrillose to arachnoid, off-white to pinkish white (7A2–3 to 9A2), dull, moist, cracked in places. Peridium very thin, 0.1–0.2(0.4) mm thick. Hymenophore cartilaginous, completely enclosed, whitish orange to reddish orange (7A3–4 to 8A5–6) at first becoming orangey red to red (9D–E8 to 10D–E8) with age, then dark red when very old, irregular; Stipe-columella absent. Taste fungoid. Odour absent when young, very strongly fruity alcoholic when old.

Macrochemical reactions: hymenophore turned dark purplish (15F8) to greyish violet (19D3) with 5% KOH, slightly greyish violet (19D3) with NH₄OH.

Basidiospores [404/8/8] (11.5–)12–13.6–15(–16) × (10–)10.5–11.7–13(–14), Q = (1–)1.04–1.16–1.3(–1.4), from the holotype, (12–)12–13.5–15.2(–16) × (10–)10–11.6–13.2(–14) µm, Q = (1–)1–1.02–1.33(–1.4), N = 50, ellipsoid to broadly ellipsoid with longitudinal ridges, stellate in polar-view, thick-walled (1–1.5 µm thick), yellowish to orangey hyaline to reddish yellow at first, reddish to brownish yellow with age in water, slightly purplish and slightly more reddish to brownish in 5% KOH, slightly purplish hyaline in NH₄OH, slightly dextrinoid to dextrinoid in Melzer’s reagent; ornamentation (7)8–9 solid ridges regularly and longitudinally arranged under light microscope, sometimes anastomosing under SEM, 2–3 µm tall and 2–2.5 µm wide at the base; hilar appendage prominent, 2.5–5 µm long with a terminal hilum. Basidia 4–spored, (26–)26.1–32.3–36(–36) × (8–)8–9.5–11(–11) µm (n = 20; from holotype only), clavate to cylindrical, hyaline in water, 5% KOH and NH₄OH, yellowish hyaline in Melzer’s reagent; sterigmata broken by spore release, stout, 3–4 µm long. Cystidia none observed. Hymenophoral trama 60–130 µm thick, irregular, with a narrow, central layer of subparallel to loosely interwoven, 3–7(8) µm wide, thin-walled hyphae, slightly gelatinised, hyaline in water, 5% KOH and NH₄OH. Peridiopellis a tomentum 45–120 µm thick, poorly differentiated, composed of thin-walled, 3–10 µm wide hyphae, anastomosing at places and covered with yellowish brown incrustations on the surface at places, otherwise hyaline in water, 5% KOH and NH₄OH, inamyloid. Clamp connections not seen in any of the tissues.

Subepigeal, solitary to gregarious (4–7 basidiomata), or fasciculate by 2–5 basidiomata, on sandy soil in dipterocarp forest dominated by Dipterocarpus tuberculatus, D. intricatus, D. obtusifolius, Shorea obtusa, S. siamensis and Eucalyptus sp.

Currently found only from Ubon Ratchathani province, northeastern Thailand.

Rhodactina rostratispora.—THAILAND, Ubon Ratchathani Province, Trakan Phuet Phon District, Don Khok Tam Lae community forest, 15°35'40.2"N–105°06'37.8"E, elev. 150 m., 28 July 2015, S. Vadthanarat 169, (CMUB, BR); ibid. 15°35'41.5"N–105°06'35.4"E, elev. 150 m., 28 July 2015, O. Raspé 1055, (CMUB, BR); ibid. 15°35'48.3"N –105°06'35.9"E, elev. 150 m., 6 August 2015, S. Vadthanarat 206, (CMUB, BR); ibid. 15°35'52.4"N–105°06'41.2"E, elev. 150 m., 6 August 2015, S. Vadthanarat 208, (CMUB, BR); ibid. 15°35'56.1"N–105°06'38.9"E, elev. 150 m., 6 August 2015, S. Vadthanarat 212, (CMUB, BR); ibid. 15°36'2.6"N–105°06'36.7"E, elev. 150 m., 14 May 2017, S. Vadthanarat 376, (CMUB, BR); Ban Huay Fai community forest, 15°32'42.7"N–105°10'16.3"E, elev. 160 m., 15 July 2017, S. Vadthanarat 406, (CMUB, BR).

R. himalayensis. – THAILAND, Chiang Mai Province, Doi Suthep-Pui National Park, forest behind Channel 9 TV station, 4 August 2000, Saisamorn Lumyong, Pipob Lumyong, Rarunee Sanmee and B. Dell 2254 (CMU25117).

R. incarnata. – THAILAND, Chiang Mai Province, Sanpatong District, Mae Wang, Conservation forest, Sanpatong-Ban Guard Rd., 24 July 2002, Saisamorn Lumyong, Pipob Lumyong, Rarunee Sanmee and Zhu L. Yang 45209 (CMU25116; holotype).

Rhodactina rostratispora is characterised by its basidiospores having a markedly prominent hilar appendage (2.5–5 µm long, 3.5–5 µm wide), with a terminal hilum; ornamentation consisting of (7)8–9 longitudinal ridges, and (11.5–)12–13.6–15(–16) × (10–)10.5–11.7–13(–14) µm. R. himalayensis has larger basidiospores (15–20 × 12.5–18 µm) without prominent hilar appendage, with fewer [(5)6–7(8)], broader ridges, while R. incarnata has a similar spore size (10–13 × 10–12 µm) and the same number of spore ridges [(7)8–9(10)] as the new species, but it does not have the prominent hilar appendage.

In one R. rostratispora specimen (S. Vadthanarat 208), abnormal spores were observed. Those spores were elongated, 21–24 × 4–8 µm, thick-walled, narrowly fusiform to bacilliform, with or without longitudinal ridges, more or less constricted in the middle. They were usually found attached to apparently normal basidia with four sterigmata.