Taxonomic and Phylogenetic Resolution of Novel Endophytic Arthrinium-like Fungi with an Updated Checklist of Nigrospora Species

Arthrinium-like fungi in the family Apiosporaceae are taxonomically complex and still require a thorough characterization despite recent phylogenetic reassessments. This study aimed to investigate the diversity and taxonomic position of endophytic Arthrinium-like fungi associated with Itea japonica and I. riparia in Thailand. Two fungal strains discovered from healthy stems of these hosts were characterized by integrative approaches including morphology, multi-locus phylogenetic analyses based on ITS, LSU, TEF1-α, TUB2 sequence data, and nucleotide base–pair comparisons. One isolate from I. japonica is introduced as Nigrospora iteae sp. nov. supported by distinct morphological traits, a well-resolved phylogenetic placement, and significant nucleotide difference from its closest relatives. The second isolate was identified as Apiospora vietnamensis and is reported herein as a new host record for I. riparia based on morphological congruence, a close phylogenetic relationship, and TUB2 nucleotide similarity with the type strain. In addition, a new species, Apiospora fici, originally described from dead leaves of Ficus septica in Taiwan, is reclassified based on updated phylogenetic analyses to clarify its taxonomic placement within Apiosporaceae. Furthermore, Nigrospora wurfbainiae nom. nov. is proposed as a replacement name for the later homonym N. guangdongensis. A summary of important morphological characteristics, host relationships, current distribution, and biological activities of Nigrospora species is provided. This study emphasizes the previously unrecognized fungal diversity within Itea hosts and offers new insights into species diversity and phylogenetic relationships within the Apiosporaceae.