Morphological and phylogenetic characterization of two <i>Rosellinia</i> subgenus <i>Corrugata</i> species causing disease on conifers in temperate forests of the United States

In 2023, conifers with Rosellinia infections were discovered in Maine, New Hampshire, and Ohio. The trees displayed twig and needle blight with dense mats of mycelium. The isolates causing the infections were provisionally identified as Rosellinia mycophila or R. herpotrichoides according to current morphology-based criteria. Both species belong to the R. thelena subgroup (subg. Corrugata) of Rosellinia as defined by Petrini (2013, Bibliotheca Mycologica 205). To date, no molecular data are available for species within this subgenus. To address this gap, we sequenced several loci and conducted a multilocus phylogenetic analysis (nuc ITS1-5.8S-ITS2 rDNA [ITS], nuc 28S rDNA D1–D2 [28S] regions, β-tubulin [TUB2], and the second-largest subunit of the DNA-directed RNA polymerase II [RPB2]). Recently, several Rosellinia species were segregated to Dematophora because they group in a separate clade and form distinct asexual morphs: Rosellinia species form Geniculosporium-like asexual states, whereas Dematophora species form Dematophora-like asexual states. Our analysis shows that R. mycophila and R. herpotrichoides form a well‑supported clade adjacent to these groups and share Nodulisporium-like anamorphs. The specimens collected in this study exhibited more than 99.3% sequence similarity. In 1961, Petrak (Sydowia 15:242–246) considered a specimen later identified as R. mycophila to be a form of R. herpotrichoides with small ascospores. In 1986, Francis (Trans Br Mycol Soc 87:397–400) disagreed and argued that the two should be maintained as separate species. Our molecular data suggest limited genetic distinction between these species; however, we do not formally synonymize the taxa. Broader sampling, especially from European and type-associated material, will be needed to evaluate whether morphological features reliably delimit R. mycophila and R. herpotrichoides.