Delimitation of Ophiostoma quercus and its synonyms using multiple gene phylogenies

Ophiostoma quercus is a morphologically variable species that causes sapstain on mostly hardwood hosts worldwide. Several species have been suggested as synonyms of O. quercus in the past, including O. fagi, O. roboris, O. valachicum, O. kubanicum, and O. catonianum. A recent collection of isolates resembling O. quercus from Azerbaijan provided the opportunity to reconsider the accuracy of these synonymies based on morphology and DNA sequence data. Four gene regions, the ribosomal internal transcribed spacer regions including the 5.8 s gene, part of the beta-tubulin gene, translation elongation factor-1 alpha, and histone gene, were used to determine the phylogenetic relationships between the various species and isolates of different origin. In all four resulting phylogenetic trees, isolates of O. quercus, O. fagi, and O. roboris formed a single, well-supported cluster, but with some internal variation. All the other species in the analyses, including O. piceae and O. catonianum, grouped distinctly with good node support. These results thus support the synonymy of O. fagi and O. roboris with O. quercus, and confirm that O. piceae and O. catonianum are distinct taxa. Ophiostoma valachicum and O. kubanicum could not be considered due to the absence of cultures, but based on published descriptions, we argue that O. valachicum should be regarded as a valid species in need of neotypification. Ophiostoma kubanicum was never validly described and should be excluded from the list of synonyms of O. quercus.