Aylward, JannekeVisagie, Cobus M.Roets, FrancoisWingfield, Brenda D.Wingfield, Michael J.2026-03-162026-03-162026-02Aylward, J., Visagie, C.M., Roets, F. et al. Genome analyses reveal two novel species of Seiridium from Acacia mearnsii. Mycological Progress 25, 8: 1-13 (2026). https://doi.org/10.1007/s11557-026-02121-7.1617-416X (print)1861-8952 (online)10.1007/s11557-026-02121-7http://hdl.handle.net/2263/109013DATA AVAILABILITY : Sequence data generated and used in this study have been deposited at GenBank under NCBI BioProject PRJNA1005571 and the accessions listed in Table S1. Alignments are provided as supplementary files.Seiridium is a genus of Sordariomycetes (Amphisphaeriales, Sporocadaceae), primarily known for species that cause cypress canker on Cupressaceae trees. However, most species in this genus have been reported from angiosperms, including many species in the native range of their hosts. Several unidentified Seiridium strains, collected > 20 years ago from Acacia mearnsii trees in eastern South Africa and southeastern Australia, were recently recovered from a collection of preserved cultures. We considered the phylogenetic position of these Seiridium strains and assessed their pathogenicity on A. mearnsii. Maximum likelihood analysis of three concatenated gene regions revealed four well-supported clades. Two closely related clades corresponded to Australian and South African origins, whereas two other clades representing South African isolates formed a monophyletic group with S. kartense, a species known from Eucalyptus cladocalyx on Kangaroo Island in Australia. Genome-wide average pairwise nucleotide identity and genetic differentiation analysis supported three species, including isolates considered conspecific with S. kartense and two that we consider to represent new species. These are described and named here as Seiridium mearnsii sp. nov. and Seiridium rouxiae sp. nov. Inoculations with South African isolates failed to produce evidence of pathogenicity. Seiridium species from A. mearnsii have likely been introduced into South Africa from Australia, along with the planting material used to establish A. mearnsii forestry.en-US© The Author(s) 2026. Open Access. This article is licensed under a Creative Commons Attribution 4.0 International License.New taxaAngiospermCupressaceaePathogenicitySingle nucleotide polymorphism (SNP)Article