UC Davis

Knowledge of pathogen biology and genetic diversity is a cornerstone of effective disease management, and accurateidentification of the pathogen is a foundation of pathogen biology. Species names provide an ideal framework for storageand retrieval of relevant information, a system that is contingent on a clear understanding of species boundaries andconsistent species identification. Verticillium, a genus of ascomycete fungi, contains important plant pathogens whosespecies boundaries have been ill defined. Using phylogenetic analyses, morphological investigations and comparisons toherbarium material and the literature, we established a taxonomic framework for Verticillium comprising ten species, five ofwhich are new to science. We used a collection of 74 isolates representing much of the diversity of Verticillium, andphylogenetic analyses based on the ribosomal internal transcribed spacer region (ITS), partial sequences of the proteincoding genes actin (ACT), elongation factor 1-alpha (EF), glyceraldehyde-3-phosphate dehydrogenase (GPD) and tryptophansynthase (TS). Combined analyses of the ACT, EF, GPD and TS datasets recognized two major groups within Verticillium, CladeFlavexudans and Clade Flavnonexudans, reflecting the respective production and absence of yellow hyphal pigments. CladeFlavexudans comprised V. albo-atrum and V. tricorpus as well as the new species V. zaregamsianum, V. isaacii and V.klebahnii, of which the latter two were morphologically indistinguishable from V. tricorpus but may differ in pathogenicity.Clade Flavnonexudans comprised V. nubilum, V. dahliae and V. longisporum, as well as the two new species V. alfalfae and V.nonalfalfae, which resembled the distantly related V. albo-atrum in morphology. Apart from the diploid hybrid V.longisporum, each of the ten species corresponded to a single clade in the phylogenetic tree comprising just one ex-typestrain, thereby establishing a direct link to a name tied to a herbarium specimen. A morphology-based key is provided foridentification to species or species groups.