Simplicillium mekongense Yao Wang & Hui Chen 2026, sp. nov.

Simplicillium mekongense Yao Wang & Hui Chen sp. nov. Fig. 3 Etymology. The epithet mekongense refers to the Lancang – Mekong River basin, where the type strain was isolated. Type. China • Yunnan Province, Pu’er City, Lancang Lahu Autonomous County (22°32'N, 99°55'E, 1147 m above sea level), isolated from soil, August 2023, Yao Wang (holotype: GMB 3232, dried culture; ex-holotype living culture: GMBC 3232). Description. Sexual morph. Undetermined. Asexual morph. Hyphomycetous. Mycelium hyaline, smooth-walled, sparsely septate. Phialides 34.8–107.2 × 1.2–2.0 μm (x ̄ = 62 × 1.5 μm, n = 30), arising from procumbent hyphae or synnemata, blastic, enteroblastic, phialidic, monophialidic, discrete, terminal, unbranched, solitary, aseptate, hyaline, smooth-walled, slender, tapering towards the apex, occasionally with a swollen node. Conidia 2.8–4.0 × 1.5–2.5 μm (x ̄ = 3.3 × 1.9 μm, n = 30), globose to ellipsoidal, hyaline, aseptate, smooth-walled, aggregated in small globose slimy heads at the apices of phialides. Culture characteristics. Colonies on PDA attaining a diameter of 4.8–5.3 cm (x ̄ = 5.1 cm, n = 10) after 14 days at 25 ° C, circular with entire margins, white and velvety on the surface, with a depressed center and radial cracks extending from the center; reverse pale yellow to yellow. Substrate. Soil. Habitat and distribution. Known from soil in subtropical regions of China (Yunnan Province), Laos (Oudomxay Province), and Thailand (Chiang Mai Province), at elevations of 330–1147 m above sea level (this study). Additional material examined. Laos • Oudomxay Province, Nam Kat Yorla Pa Resort (20°41'N, 101°59'E, 760 m above sea level), isolated from soil, August 2024, Yao Wang (GMB 3233; living culture: GMBC 3233); Thailand • Chiang Mai Province, Maejo University (18°54'N, 99°01'E, 330 m above sea level), isolated from soil, August 2023, Yao Wang (GMB 3234; living culture: GMBC 3234). Notes. Phylogenetic analyses placed Simplicillium mekongense as the sister taxon to S. subtropicum. The morphological differences between these two species are discussed below. (see Fig. 1). The species displays the diagnostic generic features of solitary, unbranched terminal phialides and conidia aggregated in slimy heads. It is morphologically similar to S. subtropicum, but its phialides are longer at the upper end of the measurement range (34.8–107.2 μm vs. 20–42 μm), although the ranges overlap, and it has larger conidia (2.8–4.0 × 1.5–2.5 μm vs. 1.5–2.5 × 1.1–1.8 μm) (Nonaka et al. 2013). Compared with S. pseudocercosporicola, S. mekongense produces markedly longer phialides (34.8–107.2 μm vs. 16.3–44.5 μm) (Custódio and Pereira 2025). From the closely related S. ovoideum, it is distinguished by longer phialides and globose to ellipsoidal conidia (vs. shorter phialides and ovoid to elongated-ovoid conidia in S. ovoideum), See Table 2 for morphological comparisons with closely related species. Ecologically, S. mekongense and S. subtropicum are soil-borne, whereas S. pseudocercosporicola is fungicolous on Pseudocercospora fijiensis, and S. ovoideum is soil-borne and also mycoparasitic on Hemileia vastatrix (Nonaka et al. 2013; Custódio and Pereira 2025). The congruent phylogenetic, morphological, and ecological evidence supports the recognition of S. mekongense as a distinct new species.