Isthmomyces oxysporus Z. F. Yu, M. Qiao & R. F. Castañeda, sp.nov. Figs 6, 9e

MycoBank number: MB 556127; Index Fungorum number: IF 556127; Facesoffungi Number: FoF 05741

Etymology: Greek, oxys, meaning sharp, keen, sporum, referring to the conidia.

Description: Asexual morph hyphomycetous. Colonies on CMA pale mouse grey to dark mouse grey, reverse olivaceous-grey, attaining about 2 cm diam. after 20 days at 25 °C. Mycelium mostly immersed, composed of branched, septate, subhyaline to hyaline hyphae. Conidiophores macronematous, mononematous, cylindrical, erect, smooth, 0–1-septate, subhyaline to hyaline, mostly reduced to conidiogenous cells, up to 30 μm long, 2.5–3 μm wide, arising from the creeping hyphae. Conidiogenous cells polyblastic, cylindrical, denticulate, integrated, terminal, sympodial extended, hyaline. Conidia isthmospore, fusiform, hyaline, smooth, 20.5–25.5 μm long, strongly constricted at the narrow, tiny central isthmus, composed of two cellular isthmicsegments: i) basal isthmic-segment broadly clavate to clavate, unicellular, hyaline 9.7–13 × 2.0–4.0 μm; ii) apical isthmic-segment narrow obclavate to obclavate, obpyriform or rarely lecythiform, unicellular, hyaline, 9.0–13.0 × 2.0–3.0 μm. Sexual state: unknown.

Type: CHINA, Hainan Province, Diaoluo Mountain Natural Reserve, on submerged leaves, August 2015, J. Peng. Holotype YMF 1.04513, preserved in a metabolically- inactive state (deep freezing) in the Conservation and Utilization of Bio-Resources in Yunnan. Ex-type culture CGMCC 3.18821.

Notes: – Morphologically, Isthmomyces oxysporus resembles Isthmolongispora asymmetrica Aramb. & Cabello in having both tapering isthmic-segment ends, but Is. asymmetrica has asymmetrical conidia, in which the basal isthmic-segment is longer (17– 20 μm long) (Arambarri et al. 1987). Besides, I. oxysporusis is somewhat similar to Is. rotundata Matsush. in conidial sizes, but the apical isthmic-segments in Is. rotundatus are rounded at the tip (Matsushima 1987).

Figure 6. Isthmomyces oxysporus (Holotype YMF 1.04513) a conidia b conidiophores and conidiogenous cells. Scale bars: 10 μm (a, b).

Figure 9. Cultural characters of all species in this study after 20 days on PDA at 25 °C.