The name refers to the location “Xishuangbanna, Yunnan, China”, where the holotype was collected.

ZHKU 23–0107.

Saprobic on a dead branch of Heveabrasiliensis. Sexual morph: Ascomata 270–410 μm high, 370–480 μm diam., (x‾ = 350 × 420 μm, n = 5), solitary, scattered, immersed, globose to subglobose, uni-loculate, black. Peridium 40–90 μm wide, thin-walled, composed of several layers of small, brown to pale brown cells of textura intricata. Hamathecium comprises 2–3 μm wide, numerous, dense, filiform, unbranched, hyaline, cellular pseudoparaphyses. Asci 130–180 × 25–35 μm (x‾ = 155 × 32 μm, n = 20), 8-spored, bitunicate, obovoid, short distinct pedicel with conical end, apex rounded with a minute ocular chamber. Ascospores 30–50 × 10–20 μm (x‾ = 42 × 13 μm, n = 30), hyaline, fusiform, with pointed ends, 3–5-septate, larger upper third cell, constricted at the septa, guttulate, thick-walled, with mucilaginous sheath, the sheath constricted at the middle. Asexual morph: Undetermined.

Colony on PDA, colonies slow growing on 28 °C, umbonate, filiform, smooth, edges brown, from above, brown, dark brown on reverse side.

China, Yunnan Province, Xishuangbanna on a dead branch of Heveabrasiliensis, 12 September, 2021, Rui-Fang Xu, XSBNR–41 (ZHKU 23–0107, holotype); ex-type ZHKUCC 23–0804, ZHKUCC 23–0805.

ZHKUCC 23–0804 = ITS: OR853097, LSU: OR922338, SSU: OR922344, tef1-α: OR966286; ZHKUCC 23–0805 = ITS: OR853098, LSU: OR922339, SSU: OR922345, tef1-α: OR966287.

In the phylogenetic analyses, Pseudochaetosphaeronemaxishuangbannaensis clusters with P.lincangensis with 99% ML and 1.00 PP support (Fig. 3). Morphologically, P.xishuangbannaensis differs from P.lincangensis in having longer asci (130–180 μm vs. 90–145 μm), 3–5-septate ascospores with sheath constricted at the central septum and brown to dark brown colonies, while P.lincangensis has ascospores with a normal sheath in a circle, 1-septate ascospores with obtuse ends and colonies off-white from the forward edge, orange in reverse. Pseudochaetosphaeronemaxishuangbannaensis shares similar morphologies with P.chiangraiense, but can be differentiated by having the peridium with the cells of textura intricate, larger ascomata (270–410 × 370–480 μm vs. 190–255 × 190–200 μm), longer asci (130–180 μm vs. 50–110 μm), larger (30–50 × 10–20 μm vs. 20–45 × 15–30 μm) and 3–5 septate ascospores with a sheath constricted at the central septum and brown to dark brown colonies. Pseudochaetosphaeronemachiangraiense has textura angularis peridium, ascospores surrounded by a normal sheath in a circle, 1-septum, obtuse ends, from above, greenish-grey in the middle and pale brown at the margin, yellowish-brown on the reverse side (Boonmee et al. 2021). In addition, P.xishuangbannaensis formed a different lineage with P.chiangraiense (Fig. 3). Therefore, P.xishuangbannaensis is described as a new species, based on phylogenetic analyses and morphological comparison.