Karstenula rhodostoma (Alb. & Schwein.) Speg., Decades Mycologicae Italicae no. 94. (1879)

Basionym: Sphaeria rhodostoma Alb. & Schwein., Consp. fung. (Leipzig): 43 (1805).

Index Fungorum number: IF 227846; MycoBank number: MB 227846; Facesoffungi number: FoF 00043

Saprobic on dead stems interrestrial habitats. Sexual state: Ascomata 250–430 × 450–650 μm ( = 310 × 500 µm, n = 10), scattered or gregarious, immersed in the subiculum which sometimes sloths off, globose or subglobose, black, flattened top often white or reddish and sometimes slightly protruding out of the substrate surface, usually with a wide opening ostiole after removing the cover, coriaceous. Peridium 30–40 μm wide, comprising two cell types, outer region single-layered, composed of relatively small heavily pigmented thick-walled compressed cells, inner layer cells larger and wall thinner, comprising cells of textura angularis, merging with pseudoparaphyses. Hamathecium of dense, 2–3.5 μm wide, long cellular, septate pseudoparaphyses. Asci 150–210 × 12–15 μm (= 182 × 13 μm, n = 20), 8-spored, bitunicate, fissitunicate, cylindrical, with a broad, furcate pedicel which is 12–35 μm long, and with an ocular chamber. Ascospores 20–26 × 7.5–10 μm (= 22 × 8 μm, n = 20), obliquely uniseriate and partially overlapping, ellipsoid, reddish-brown, with 3-transverse septa and a vertical septum in one or two central cells, constricted at the septa, verruculose. Asexual state: unknown.

Material examined – SWEDEN, on dead stems, 1834, E. Fries (PH 01048835, holotype of Sphaeria rhodostoma Alb. & Schwein.).

Fig. 1 Karstenula rhodostoma (PH 01048835, holotype of Sphaeria rhodostoma). a-b Herbarium packet and specimen. c. Ascomata on substrate. d Section thought ascoma. e Asci arrangement. f Close-up of the peridium. g Hamathecium comprising cellular pseudoparaphyses. h–i Asci when immature. j Ascus at maturity. k–m Ellipsoid, reddish-brown ascospores. Scale bars: d = 100 µm, e, h–j = 50 µm. f, k–m = 20 µm, g = 10 µm.

Notes – Karstenula is an ambiguous genus, which has been synonymised under Pleomassaria by several authors (Lindau 1897; Winter 1885). Karstenula shares similarities with Didymosphaeria in having ascomata seated in a subiculum or beneath a clypeal thickening and sometimes apical cells become reddish or orange-brown (Barr 1990). Barr (1990) modified the concept of Karstenula (sensu lato), which encompasses some species of Thyridium. However, Barr (1990) treated Karstenula as having trabeculate pseudoparaphyses and this is clearly not the case. In most cases, the ascospores are brown with transverse septa and sparse longitudinal septa. The ascomata of Karstenula rhodostoma are similar to those found in Byssosphaeria and Herpotrichia, especially in the paler area around the ostiole and even in peridium and development under a subiculum. The numerous wide cellular pseudoparaphyses and cylindrical asci (in Herpotrichia) are also similar. The main difference between Karstenula and the other two genera is the 3-septate ascospores with rare longitudinal septa (1-septate in Byssosphaeria and Herpotrichia).

Recent phylogenetic studies have shown that the putative strain of Karstenula rhodostoma (CBS 690.94) forms a robust clade with Phaeodothis winteri (Niessl) Aptroot, Didymocrea sadasivanii, Bimuria novae-zelandiae, Montagnula opulenta, Curreya pityophila (J.C. Schmidt& Kunze) Arx & E. Müll., and some species of Letendraea and Paraphaeosphaeria (Kodsueb et al. 2006a; Zhang et al. 2009a). Thus Zhang et al. (2012) tentatively included Karstenula in Montagnulaceae.

Our phylogenetic analysis reveals that the putative strain of Karstenula rhodostoma (CBS 690.94) resides in Didymosphaeriaceae, sister to the Paraphaeosphaeria clade. Therefore we tentatively refer Karstenula to Didymosphaeriaceae. However, the placement of Karstenula in Didymosphaeriaceae can only be confirmed by phylogenetic work including sequencing the generic type of Karstenula (K. rhodostoma).