Phylogenetics and Taxonomy of the Fungal Vascular Wilt Pathogen Verticillium, with the Descriptions of Five New Species

Abstract

Knowledge of pathogen biology and genetic diversity is a cornerstone of effective disease management, and accurate identification of the pathogen is a foundation of pathogen biology. Species names provide an ideal framework for storage and retrieval of relevant information, a system that is contingent on a clear understanding of species boundaries and consistent species identification. Verticillium, a genus of ascomycete fungi, contains important plant pathogens whose species boundaries have been ill defined. Using phylogenetic analyses, morphological investigations and comparisons to herbarium material and the literature, we established a taxonomic framework for Verticillium comprising ten species, five of which are new to science. We used a collection of 74 isolates representing much of the diversity of Verticillium, and phylogenetic analyses based on the ribosomal internal transcribed spacer region (ITS), partial sequences of the protein coding genes actin (ACT), elongation factor 1-alpha (EF), glyceraldehyde-3-phosphate dehydrogenase (GPD) and tryptophan synthase (TS). Combined analyses of the ACT, EF, GPD and TS datasets recognized two major groups within Verticillium, Clade Flavexudans and Clade Flavnonexudans, reflecting the respective production and absence of yellow hyphal pigments. Clade Flavexudans 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-type strain, thereby establishing a direct link to a name tied to a herbarium specimen. A morphology-based key is provided for identification to species or species groups.

Verticillium dahliaeand other pathogenic fungi inCirsium arvensefrom New Zealand pastures: occurrence, pathogenicity and biological control potential

“…However, the morphological characteristics (e.g. absence of yellow pigmentation, microsclerotia as resting structures and conidial length generally B6 mm) of the fungus found in our surveys correspond to those of published descriptions of V. dahliae (Smith 1965;Inderbitzin et al 2011). Furthermore, there are other published reports of the isolation of V. dahliae from C. arvense in Europe (Leth & Haas 1984;Henriksson 1995;Demirci & Genc 2009) and New Zealand (Waipara et al 1991;Dodd et al 2010).…”

“…The fungus has also been detected in C. arvense in New Zealand by PCR methods ) using primers specifically targeting the V. dahliae b tubulin gene (Atallah et al 2007), or by closest sequence match with V. dahliae following a GenBank BLASTn search (Dodd et al 2010). Recent research has indicted that, while plant pathogenic species of Verticillium can be distinguished by their morphological characters in culture (Goud et al 2003), the phylogenetic relationship among isolates is complicated (Qin et al 2006;Inderbitzin et al 2011). Future studies on Verticillium spp.…”

Scientific opinion on the pest categorisation of Verticillium dahliae Kleb

“…More recently, Inderbitzin et al (2011) wanted to create a solid taxonomic framework for Verticillium, their evolutionary relationships and the species boundaries using multigene phylogenetic analyses and morphological investigations. The description of V. dahliae agreed with the original description by Klebahn, except that Inderbitzin et al (2011) failed to detect strands of erect, hyphal aggregates containing conidia and microsclerotia.…”

“…Aerial hyphae are smooth-walled, (1.5-) 2-4 µm wide, at times containing inflated cells up to 9 µm wide. Conidia are hyaline, smooth-walled, non-septate, cylindrical with rounded apices to oval, allantoid or tapering at times, (3.5-) 6.5 µm ± 1.5 µm (-13.5) × (2.0-) 3.0 µm ± 0.5 µm (-4.5 (Inderbitzin et al, 2011).…”

First report of the presence of Verticillium dahliae VCG1A in Australia

“…systems worldwide (EFSA 2014;Inderbitzin et al 2011;Pegg and Brady 2002). The problems caused by V. dahliae are exacerbated by its extensive and expanding host range (Subbarao et al 1997) and its ability to survive in soil by means of microsclerotia, in the absence of a host, for up to 14 years (Wilhelm 1955).…”