Three new fungal species, Septobasidium macrobasidium, S. puerense and S. wuliangshanense, are proposed based on a combination of the morphological features and molecular evidence. The taxon S. macrobasidium is characterized by the coriaceous basidiomata with a cream surface, cylindrical basidia, straight, 4-celled, subglobose or ovoid probasidia and thin-walled, narrowly cylindrical basidiospores with septa, measuring as 7–9 × 3.5–4.5 µm, the haustoria consisting of irregularly coiled hyphae; in addition, this fungus was found associated with the insect of Diaspididae. The species S. puerense is characterised by resupinate coriaceous basidiomata with a cinnamon brown to chestnut brown surface, cylindrical or slightly irregular basidia, 2-3-celled, slightly curved, subglobose to pyriform probasidia, probasidia cell persistent after the formation of the basidia and the haustoria with two types consisting of irregularly coiled hyphae and spindle-shape. The fungus was found associated with the insect species Pseudaulacaspis pentagona. The species S. wuliangshanense is characterised by the coriaceous basidiomata with a slightly brown surface, cylindrical or slightly irregular basidia, 2-3-celled, straight or slightly curved, pyriform, subglobose or ovoid and probasidia, haustoria consisting of irregularly coiled hyphae, associated with the insect genus Aulacaspis. Sequences of internal transcribed spacer region (ITS) were analysed maximum likelihood, maximum parsimony and Bayesian inference methods. The new species S. macrobasidium was clustered with S. maesae. Furthermore, S. puerense was retrieved as a sister to S. carestianum. The phylogenetic tree, inferred from the ITS sequences, highlighted that S. wuliangshanense was the sister to S. aquilariae with strong supports. Application of PHI test to the ITS tree-locus sequences revealed no recombination level within phylogenetically related species.

Forest disease, phylogenetic analyses, taxonomy, wood-inhabiting fungi, Yunnan Province

Fungi are a diverse, monophyletic group of eukaryotes and these organisms show immense ecological and economic impacts for playing an important role in the ecosystems as diverse as soil, trees, with hidden layers within their substrate (James et al. 2020). Approximately 150 thousand species of fungi have been described (Cui et al. 2019; Dai et al. 2021; Wang and Zhao 2021; Sun et al. 2022; Liu et al. 2023). But the potential biodiversity of the group is likely to be 2.2–3.8 millions of species (Blackwell 2011; Taylor et al. 2014; Hibbett 2016; Dong et al. 2023). The diversity for flora of seed plants in Yunnan Province is higher than that in other areas of China. The number of discovered new fungal species totalled 1,345 from this province from 2000 to 2020 years. Endemic woody plants are rich in Yunnan, supplying rich and varied substrates for wood-decaying fungi. The pathogenic fungi with industrial, medicinal and economic value, comprise most basidiomycetes and ascomycetes growing on various kinds of living trees (Dai et al. 2015, 2021; Vinay et al. 2015; Wu et al. 2019; Luo et al. 2022).

Mutualistic and parasitic symbioses between fungi and plants are widely acknowledged to have profound influences on the evolution and ecology of terrestrial life, but less well-known are the symbioses between fungi and insects (Henk and Vilgalys 2007). The insects are protected from their enemies by the fungus and the fungus draws its nutriment from, and is distributed by, the insects. Under natural conditions, whereas the fungus cannot live without the insects, the insects are able to live without the fungus, but the life of insects unprotected by the fungus is precarious (Couch 1938).

Basidiomycete fungi have evolved many symbiotic associations with plants and animals, but fungi in the order Septobasidiales are the only large group of basidiomycetes that are obligately parasitic on insects. Understanding the evolution of insect parasitism and switches from plant parasitism in the Basidiomycota requires a phylogeny to place the order Septobasidiales within the Pucciniomycotina and to determine whether the different forms of insect parasitism in the order Septobasidiales and Septobasidium Pat. have a single origin. Septobasidium is a type of fungi that has a mutualistic relationship with insects. Although Septobasidium sterilize the individuals they parasitize, the fungi may protect uninfected individuals and thereby benefit the population of scale insects (Couch 1938). All fungi in the order Septobasidiales do not display this type of symbiosis. Some may be wholly parasitic because they do not form substantial protective structures. This fungus-insect symbiosis is important because of its unique altruistic and parasitic characteristics and its phylogenetic position within the Basidiomycota (Henk and Vilgalys 2007).

Septobasidium (Septobasidiaceae, Septobasidiales), erected by Patouillard (1892), typed by Septobasidium velutinum Pat., which is a large cosmopolitan genus characterised by perennial colonies on the surfaces of plant structures with colonies of scale insects, the basidiomata are usually white to cream, yellowish brown or brown hymenophore, rarely more brightly coloured, normally resupinate and felty in texture; their surfaces may be smooth, warty, or spiny (Patouillard 1892; Couch 1929); a monomitic hyphal system with simple septa, with or without probasidia, 2–4 celled cylindrical, curved, or straight basidia, basidiospores that are hyaline, thin-walled, smooth and cylindrical or fusiform and haustoria consisting of coiled or spindle-shaped hyphae (Ma et al. 2019). In a previous study, Couch (1938) divided it primarily based upon morphological characters related to the structure of the basidia and probasidia. Basidia may be curved or straight and may have one, two, three, or four cells. The probasidia either remains as an empty cell at the base of the mature basidia (persistent) or becomes a spore-bearing cell in the mature basidia (not persistent). Other characters used as indicators of major groups in Septobasidium include the layered nature of the thallus and the presence of pillar structures (Oberwinkler 1989). Septobasidium occurs on living leaves, stems and branches of a great variety of perennial plants, including gymnosperms, monocots and dicots. Inexperienced collectors may mistake them for corticioid basidiomycetes or even lichens (Couch 1929, 1935, 1938, 1946).

As with most basidiomycetes, basidia are produced so that they project toward the ground. Thus, the resupinate basidiomata often are found on the lower sides of branches. They occur on living rather than dead plant parts and away from the extreme tips of branches which distinguishes them from some resupinate species of Aphyllophorales (Couch 1938; Henk and Vilgalys 2007). About 305 species have been accepted into the genus worldwide (Patouillard 1892; Bresadola and Saccardo 1897; Burt 1916; Lloyd 1919; Couch 1929, 1935, 1938, 1946; Yamamoto 1956; Gómez and Henk 2004; Henk 2005; Lu and Guo 2009a, 2009b, 2010a, 2010b, 2011; Chen and Guo 2011a, 2011b, 2011c, 2012; Lu et al. 2010; Li and Guo 2013, 2014; Ma et al. 2019).

Molecular systematics has played a powerful role in inferring phylogenies within fungal groups since the early 1990s (White et al. 1990; Binder et al. 2013; Dai et al. 2015; Choi and Kim 2017). Classification of the kingdom of fungi has been updated continuously, based on the frequent inclusion of data from DNA sequences in many phylogenetic studies (Wijayawardene et al. 2020). However, molecular studies involving Septobasidium are rare (Henk and Vilgalys 2007; Zhao et al. 2017). One phylogenetic study of a single origin of insect symbiosis in the class Pucciniomycetes suggested that there is little or no support for Septobasidium as a monophyletic group (Henk and Vilgalys 2007). The previous study introduced a six-gene phylogenetic overview of Basidiomycota and allied phyla and confirmed that S. carestianum Bres. nested within the order Septobasidiales and grouped with Helicobasidium mompa Nobuj. Tanaka and Thanatophytum crocorum (Pers.) Nees (Zhao et al. 2017).

During investigations into the wood-inhabiting fungi in Pu’er, Yunnan of China, samples representing three additional species belonging to genus Septobasidium were collected. Three new Septobasidium taxa were found that could not be assigned to any described species. To clarify the placement and relationships of these specimens, we carried out a phylogenetic and taxonomic study based on the ITS sequences. A description, illustrations, and phylogenetic analysis results of the new species are provided.

Many recently described wood-inhabiting fungal taxa have been reported worldwide, including in the genus Septobasidium (Patouillard 1892; Bresadola and Saccardo 1897; Burt 1916; Lloyd 1919; Couch 1929, 1935, 1938, 1946; Yamamoto 1956; Gómez and Henk 2004; Henk 2005; Chen and Guo 2009a, 2009b, 2010, 2011a, 2011b, 2011c, 2012; Lu and Guo 2009a, 2009b, 2010a, 2010b 2011, 2013, 2014; Lu et al. 2010). The diversity of Septobasidium is rich in China. Prior to this study, the following 58 Septobasidium species were reported from China, especially in subtropics and tropics (Lu and Guo 2009a, 2009b, 2010a, 2010b, 2011; Lu et al. 2010; Chen and Guo 2011a, 2011b, 2011c, 2012; Li and Guo 2013, 2014; Ma et al. 2019). Several Septobasidium species have been described from Yunnan Province (Lu and Guo 2010b, 2011; Ma et al. 2019). In the present study, three new species, S. macrobasidium, S. puerense and S. wuliangshanense are described based on phylogenetic analyses and morphological characteristics. In addition, the PHI test (Fig. 2) was carried out to confirm that there is no recombination present in the new species S. wuliangshanense compared with closely related taxa.

Based on ITS topology (Fig. 1), the phylogenetic tree includes the type species Septobasidium velutinum, which is collected from Costa Rica and most species of this genus have persistent probasidia, except for S. aquilariae, S. gomezii Henk, S. hainanense C.X. Lu & L. Guo, S. pallidum Couch, S. septobasidioides (Henn.) Höhn. & Litsch. and S. westonii Couch. Most Septobasidium species have irregularly coiled haustoria and the spindle-shaped haustoria of S. fumigatum Burt, S. griseum Couch, S. grandisporum Couch, S. pilosum Boedijn & B.A. Steinm and S. sinuosum Couch. However, S. puerense has two types of haustoria: 1) consisting of irregularly coiled hyphae; 2) spindle-shape. Most Septobasidium species have pillars, except for S. arachnoideum (Berk. & Broome) Bres., S. burtii Lloyd, S. canescens Burt, S. cavarae, S. fumigatum, S. grandisporum, S. michelianum (Caldesi) Pat., S. pilosum, S. pinicola Snell, S. sinuosum, S. taxodii Couch and S. wilsonianum L.D. Gómez & Kisim.-Hor.

Phylogenetically, based on the ITS topology (Fig. 1), Septobasidium macrobasidium is clustered with S. maesae. The new species S. puerense is closely related to S. carestianum and S. wuliangshanense is sister to S. aquilariae with strong supports. But morphologically S. maesae differs from S. macrobasidium by its greyish-brown hymenial surface and smaller basidia (28–55 × 7.5–11.5 µm), brown, fusiform basidiospore without septa (18–19.5 × 4–5 µm; Lu and Guo 2009a). The species S. carestianum differs from S. puerense by its iuventute avellaneas deinde cinnamomeas, superficiei sub lente pruinosula hymenial surface and the larger sphaerica probasidia (15.1–11.3 µm) and 4-celled, apicem acuta, longer cylindrica basidia in senectute brunnea (62–71 × 5–6.7 µm), ellipticae and flexae sporae (21–23 × 4.2–5 µm; Gómez and Henk 2004). The species S. aquilariae differs from S. wuliangshanense by its smaller basidia (15–26.5 × 4–6 µm) and without a probasidia cell and reniform basidiospores (11–19 × 4–7.5 µm), habitat and distribution growing in association with Pseudaulacaspis sp. on Aquilaria sinensis (Lour.) Spreng. (Ma et al. 2019). Further, application of PHI test to the ITS tree-locus sequences revealed no recombination level within phylogenetically in these two species (Fig. 2).

Morphologically, Septobasidium cokeri Couch. differs from S. macrobasidium by its pure white hymenial surface and restricted growth on Quercus rubra L. (Gómez and Henk 2004). The species S. hainanense differs in its purple hymenial surface and smaller basidia (25–36 × 7–13 µm; Lu and Guo 2010a). The taxon S. maesae differs by its perennial basidiomata peeled off after maturity and smaller basidia (28–55 × 7.5–11.5 µm; Lu and Guo 2009a).

Septobasidium guangxiense Wei Li bis & L. Guo differs from S. puerense in its yellowish brown hymenial surface with numerous fissures at maturity and larger basidia (27–38 × 5–10 µm; Li and Guo 2014). The species S. hoveniae Wei Li bis, S.Z. Chen, L. Guo & Yao Q. Ye differs in its cinnamon-brown hymenial surface and growth on Hovenia acerba (Li and Guo 2013; S. polygoni C.X. Lu & L. Guo differs in its white to cinnamon-brown hymenial surface and growth on Polygonum campanulatum Hook. f. (Lu and Guo 2010b); S. reevesiae S.Z. Chen & L. Guo differs in its thicker section (1.65–2.20 mm) and larger basidia (37–55 × 8–13 µm) and growth on Reevesia longipetiolata Merr. & Chun (Chen and Guo 2012) and S. transversum Wei Li bis & L. Guo differs in its cinnamon-brown basidiomata, its transverse layer at the pillar bases and larger basidia (42–60 × 9–16 µm; Li and Guo 2014).

Morphologically, several species of Septobasidium broussonetiae C.X. Lu, L. Guo & J.G. Wei, S. brunneum Wei Li bis & L. Guo, S. capparis S.Z. Chen & L. Guo, S. euryae-groffii C.X. Lu & L. Guo, S. fissuratum Wei Li bis & L. Guo and S. gaoligongense C.X. Lu & L. Guo are similar to S. wuliangshanense were found in China. However, S. broussonetiae is distinguished by its cracking basidiomata and growth on Broussonetia papyrifera (L.) L’Hér. ex Vent. (Lu et al. 2010); S. brunneum differs in its purple-brown hymenial surface with many cracks and growth on Eurya sp. (Li and Guo 2014); S. capparis differs by its thicker section (less than 2 mm thick) and larger basidia (45–56 × 8–12 µm; Chen and Guo 2012); S. euryae-groffii is distinguished by its cinnamon to chestnut brown hymenial surface and growth on Eurya groffii (Lu and Guo 2010b); S. fissuratum differs in its larger basidia (32–45 × 6–9 µm) and growth on Castanea sp. (Li and Guo 2013); S. gaoligongense differs in its dark brown hymenial surface and thinner section (260–580 µm; Lu and Guo 2010b).

Based on our phylogenetic and morphological research results, 61 species have been reported from China, including newly described in the present study and other recently published papers in this country (Gómez and Henk 2004; Lu and Guo 2009a, 2009b, 2010a, 2010b, 2011; Lu et al. 2010; Chen and Guo 2011a, 2011b, 2011c, 2012; Li and Guo 2013, 2014; Ma et al. 2019). It seems that the species diversity of Septobasidium is rich in China. Although the taxa of Septobasidium are well studied in the present paper and the species diversity, taxonomy and phylogeny of Septobasidium and related genera are still unresolved. A comprehensive study on this issue is urgently needed.