Fungal genus Scytalidium narrowed to 16 related species

A revision of the fungal genus Scytalidium found that accepted species had been scattered across at least four classes of ascomycete fungi. Researchers combined colony and spore morphology with molecular analyses of the internal transcribed spacer and large-subunit ribosomal DNA regions, and restricted the genus to a monophyletic group of 16 species containing the type species, Scytalidium lignicola. Three new soil-associated species were described from chilli pepper cultivation areas in Guizhou Province, China, while several names were transferred to other genera, synonymized or classified as uncertain.

The study was based on annual soil sampling conducted from 2022 through 2024 in three chilli pepper-growing regions of Guizhou: Liuguang Town in Xiuwen County, Shiban Town in Huaxi District, and Qiaojia Town in Yanhe County. Samples were taken from 0–10 cm below the surface and processed by dilution methods. The researchers obtained seven isolates of Scytalidium and cultivated them on potato dextrose agar and synthetic nutrient-poor agar. Colony characteristics were assessed after 14 days at 25 °C in darkness, while microscopic structures were examined with light microscopy and measured digitally.

For the broad taxonomic analysis, the investigators assembled concatenated internal transcribed spacer, or ITS, and large-subunit, or LSU, sequences from 87 taxa. Maximum-likelihood and Bayesian analyses placed 16 Scytalidium species in a strongly supported clade within Leotiomycetes, with 98% bootstrap support and a Bayesian posterior probability of 1. Other species and strains were distributed among Sordariomycetes, Dothideomycetes and Eurotiomycetes. This pattern indicated that Scytalidium was polyphyletic as previously circumscribed, a result that the authors say requires the genus to be narrowed around its type lineage.

A focused analysis of the restricted genus included 33 taxa and 1,400 aligned characters, comprising 581 base pairs of ITS and 819 of LSU. The resulting Scytalidium clade was divided into five principal groups. One contained S. album, S. assmuthi, S. aurantiacum, S. rodionovae, S. circinatum, S. terrigenum and S. tongrenense; another joined S. ganodermophthorum with S. synnematicum. The remaining groups included the newly described S. guizhouense, a cluster of S. chlamydosporum, S. cuboideum and S. sphaerosporum, and a clade containing S. auriculariicola, S. lignicola and S. philadelphianum. Each newly described species formed a distinct, strongly supported branch.

One of the new taxa, Scytalidium chlamydosporum, was represented by three isolates from soil in Guizhou. After 14 days on potato dextrose agar, its colonies reached 80–82 mm across and were flat and cottony, ranging from gray-yellow to pale gray. Its smooth, septate hyphae were 1–3 μm in diameter, while its yellow-brown to dark-brown chlamydospore-like cells were solitary or catenate, thick-walled and measured 6–11.5 by 6–8.5 μm. No arthroconidia or sexual morph were observed. The new species differed from S. cuboideum and S. sphaerosporum in these reproductive characteristics and in ITS sequence divergence of 10.9% and 12.4%, respectively.

A second species, Scytalidium guizhouense, was represented by two isolates from soil in Shiban Town. Its potato dextrose agar colonies reached 48–55 mm after two weeks and ranged from ash gray to dull green, whereas colonies on synthetic nutrient-poor agar were white with radially striate margins. Unlike S. chlamydosporum, it produced conidia, arthroconidia and chlamydospore-like cells, although its sexual morph remained unknown. Its arthroconidia measured 3–7.5 by 2–3 μm, while the brown, zero- or one-septate chlamydospore-like cells measured 7.5–13 by 4.5–6.5 μm. The isolates were recovered from pepper cultivation soil and are considered likely saprophytes, although that lifestyle was not directly tested.

The third new species, Scytalidium tongrenense, was described from soil in Qiaojia Town, with an additional isolate from Shiban Town. Colonies on potato dextrose agar reached only 17–34 mm across and were white, while those on synthetic nutrient-poor agar reached 66–68 mm and were pale gray. The fungus produced yellow-brown to dark-brown, zero- or one-septate chlamydospore-like cells measuring 5.5–12 by 4.5–6 μm. Arthroconidia and a sexual morph were not observed. Its two isolates formed a fully supported subclade and were sister to a larger group containing S. album, S. assmuthi, S. aurantiacum, S. rodionovae, S. circinatum and S. terrigenum.

The revision also corrected the naming of strain 3C, isolated from a rotting rope in Moscow during the 1970s. After its original identification as Geotrichum candidum and its later reclassification as Scytalidium candidum, the strain was found not to belong to G. candidum. The authors therefore proposed Scytalidium rodionovae, with strain 3C designated as the ex-type culture. It has a known sexual morph and hyaline, ovoid, septate chlamydospore-like cells, distinguishing it from several close relatives. The strain has also been reported to secrete cellulolytic enzymes and to tolerate a broad pH range, properties that make it of potential interest for lignocellulosic biomass conversion.

Phylogenetic placement also led to changes beyond the newly named species. The ex-type strain of Scytalidium tuberculatum was found within Neocosmospora, where the available epithet was already occupied, resulting in the combination Neocosmospora xizangensis. Scytalidium terminale was transferred to Hypoxylon; S. uredinicola became Caliciopsis uredinicola; and S. infestans became Neodevriesia infestans, with N. cladophorae treated as its synonym. The ex-type of S. indonesiacum clustered with Mycothermus thermophilus, showing 99% ITS and 99.5% LSU similarity, and was synonymized under that name. Likewise, S. flavobrunneum was placed with Monochaetia dimorphospora, with 99.9% ITS-LSU similarity. The ex-type of S. parasiticum clustered with S. ganodermophthorum and differed by only one base across 516 compared ITS positions, supporting treatment of the former as a synonym.

Not every formerly included name could be resolved conclusively. The ex-type of Scytalidium multiseptatum lacked molecular sequence data, while available strains placed it in a separate lineage within Tricladiaceae. The ex-type of S. chinense formed an independent Dothideomycetes lineage, and the two available strains of S. japonicum fell into different groups, one within Dothideomycetes and the other near Neoscytalidium. Six species without molecular sequences—S. fulvum, S. hepiali, S. melanoxylicola, S. nielamuense, S. verruculosum and S. xigazense—were designated incertae sedis. The authors cautioned that ITS and LSU alone do not meet all proposed standards for fungal species delimitation, which also call for at least one protein-coding gene; future revision will therefore need secondary barcodes or genome-scale data. The study provides a substantially narrower framework for recognizing Scytalidium, but the placement of several unsequenced and morphologically ambiguous taxa remains unresolved.

Subject of Research: Phylogenetic and morphological revision of the fungal genus Scytalidium

Article Title: Phylogenetic assessment and taxonomic revision of Scytalidium ( Helotiales , Leotiomycetes )

Article References: Tong, S.-Q., Yang, Y.-F., Li, P., Wu, Y.-J., Sun, B.-D., & Zhang, Z.-Y. (2025). Phylogenetic assessment and taxonomic revision of Scytalidium (Helotiales, Leotiomycetes). IMA Fungus, 16, Article e164608. https://doi.org/10.3897/imafungus.16.164608

Image Credits: AI Generated

DOI: 10.3897/imafungus.16.164608

Keywords: Scytalidium, fungal taxonomy, phylogenetics, Leotiomycetes, new species, soil fungi, ITS sequences, LSU sequences, ascomycetes, Guizhou China, Phylogenetic, assessment