Multiseptisporium aquaticum H. W. Shen & Z. L. Luo 2026, sp. nov.

Multiseptisporium aquaticum H. W. Shen & Z. L. Luo sp. nov. Fig. 2 Chinese name: 水生多隔孢 (Pinyin: ShuĬ shēng duō gé bāo) Etymology. The specific epithet “ aquaticum ” refers to the aquatic habitat where the fungus was collected. Holotype. HKAS 151655. Description. Saprobic on submerged decaying wood in freshwater habitats. Sexual morph: Undetermined. Asexual morph: Colonies on natural substrate, superficial, effuse, scattered, hairy, dark brown, solitary, or in small groups of 2–3. Mycelium partly immersed and partly superficial, composed of branched, septate, brown to dark brown, smooth-walled hyphae. Conidiophores (104 –) 120–204 (– 234) × 6–8 µm (x – = 162 × 7 µm, SD = 41.6 × 0.6, n = 15), macronematous, mononematous, cylindrical, straight or slightly flexuous, unbranched, 8–9 - septate, slightly enlarged at the base, dark brown to black, smooth-walled. Conidiogenous cells (11 –) 14–21 (– 23) × 4–6 µm (x – = 18 × 5 µm, SD = 3.0 × 0.5, n = 15), monoblastic, integrated, terminal, determinate, cylindrical, slightly narrow at the top, brown, smooth-walled. Conidial secession rhexolytic. Conidia (53 –) 57–68 (– 75) × 10–14 µm (x – = 63 × 12 µm, SD = 5.7 × 1.2, n = 15), acrogenous, solitary, straight to slightly curved, 8–10 - euseptate, obclavate to rostrate, truncated at base, tapering towards an acuminate apex, brown to dark olivaceous brown, subhyaline at apex, typically widest at third cell from base, with cuneiform basal cell, similar in color to apical cells, slightly constricted at septa, guttulate, smooth, occasionally verruculous, sometimes with an inconspicuous sheath at apex. Culture characteristics. Conidia germinating on PDA within 12 h and germ tubes produced from both ends. Colonies on PDA reaching 2–3 cm in diameter after two months at room temperature (around 22 ° C) in the dark, dense, velvety, gray to dark brown at the margins, pale to gray at the center, with irregular margins and bulge center from above; dense, gray to dark gray, with irregular margins from below. Material examined. China • Yunnan Province, Honghe Hani and Yi Autonomous Prefecture, Shiping County, Yilonghu Lake, 23°40'56.4"N, 101°43'46.8"E, elevation 1400 m, on the unknown submerged decaying wood, 23 February 2023, H. W. Shen, L 2194 (HKAS 151655, holotype), ex-type KUNCC 23-14304; • ibid. Guizhou Province, Zunyi City, Xishui County, 28°9'17"N, 105°55'50"E, elevation 640 m, on the unknown submerged decaying wood, 2 October 2023, X. J. Xiao, NJS 15 (HKAS 152855, paratype), ex-paratype GZCC 24-0036. Notes. Phylogenetic analysis revealed that Multiseptisporium aquaticum (KUNCC 23-14304 and GZCC 24-0036) clustered with Platytrachelon abietis (CBS 125235) with stable support (Fig. 1). Platytrachelon abietis, originally described as Ceratosphaeria abietis, was collected from the inner side of decaying bark of Abies alba in the Czech Republic and has also been reported from Ukraine (Réblová 2013). Based on morphological and phylogenetic evidence, Réblová (2013) transferred C. abietis to the newly established genus Platytrachelon. Both sexual and asexual morphs of P. abietis have been documented. Notably, the asexual morph, including a synanamorph, was observed on PCA culture media. The asexual form of P. abietis is characterized by spherical, hyaline to subhyaline, holoblastic conidiogenous cells that proliferate sympodially; ellipsoidal, septate, smooth-walled conidia, sometimes arising directly from subhyaline or pigmented hyphae. The synanamorph is characterized by cylindrical to ampulliform, one-celled, hyaline to subhyaline conidiogenous cells terminating in a single phialidic opening or with an additional lateral opening; the collarette is shallow, wedge-shaped, and inconspicuous; conidia are cylindrical, strongly curved, hyaline, one-celled, and smooth-walled (Réblová 2013). Multiseptisporium aquaticum differs from P. abietis in having sporidesmium-like conidia produced from monoblastic, terminal, cylindrical conidiogenous cells, with rhexolytic conidial secession. No synanamorphs were observed. Comparison of nucleotide base differences in LSU and rpb 2 sequence data between M. aquaticum (KUNCC 23-14304) and P. abietis (CBS 125235) showed differences of 3.2 % (34 / 1,071 bp, including four gaps) and 11.8 % (122 / 1,032 bp), respectively. Based on the combined morphological characteristics and sequence data, M. aquaticum is proposed as a new species.