Roussoella chiangraiensis Gomdola, Z. L. Tun & Yong Wang bis 2026, sp. nov.

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Roussoella chiangraiensis Gomdola, Z. L. Tun & Yong Wang bis sp. nov. Fig. 3 Holotype. MFLU 26-0004. Etymology. Referring to Chiang Rai Province, where the species was collected. Description. Saprobic on bamboo stems. Sexual morph: Ascomata 50–200 μm diam., superficial on the host substrate, aggregated, globose to subglobose, black. Asci 45–75 × 2.5–4 μm (x ̄ = 60.1 × 3.6 μm, n = 20), bitunicate, fissitunicate, 6–8 - spored, cylindrical to clavate, straight or slightly curved, rounded at the apex with an ocular chamber. Ascospores 6–10 × 2–3.5 µm (x ̄ = 7.9 × 2.9 µm, n = 30), uniseriate, broadly fusiform to sub-ellipsoidal, tapering towards one or both ends, mostly straight, smooth- and thick-walled, hyaline when immature, becoming pale brown to brown at maturity, 1 - median-septate, constricted at the septum, usually containing a large guttule in each cell, with longitudinal striations. Asexual morph: Not observed. Material examined. Thailand. • Chiang Rai Province: Mae Fah Luang University premises, on dead stems of bamboo, 20 November 2025, Zaw Lin Tun, TP 1 (MFLU 26-0004, holotype). GenBank numbers. ITS = PX 940742, 28 S = PX 956942, 18 S = PX 956882, rpb 2 = PZ 097585. Notes. Our strain (MFLU 26-0004) exhibits the diagnostic characters of Roussoella, including bitunicate asci, and brown, fusiform to ellipsoidal, septate ascospores (Liu et al. 2014). In the multilocus phylogeny, it forms a sister clade to R. aseptata (CGMCC 3.25604; ex-type and UESTCC 24.0151), with maximum support (100 % ML, 1.00 PP) (Fig. 1). No nucleotide differences were detected in the 18 S region between our strain and R. aseptata (CGMCC 3.25604; 622 bp and UESTCC 24.0151; 772 bp). However, excluding gaps, interspecies divergences of 3.9 % in ITS (18 / 462 bp) and 0.24 % in 28 S (2 / 845 bp) were recovered. Morphological comparison with R. aseptata was not possible, as the latter is known only from its asexual morph, whereas our strain was obtained in the sexual morph (Yu et al. 2024). Based primarily on phylogenetic analyses and the ITS divergence, we describe MFLU 26-0004 as a new species, Roussoella chiangraiensis, saprobic on bamboo stems in Thailand. It is noteworthy that our strain lacks the raised, semi-immersed to immersed ascostromata typical of many Roussoella species (Liu et al. 2014). Instead, it produces superficial, carbonaceous ascomata that could not be hand-sectioned and appear among other free fungal spores on the substrate.

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