Plant pathogenic Verticillium species: how many of them are there?

SUMMARY Two of the currently widely accepted species in the section Nigrescentia of the genus Verticillium are major plant pathogens inducing wilt diseases in a wide range of mainly dicotyledonous hosts. Three species closely related to these two are less important wilt pathogens and soil saprophytes. A sixth species, V. theobromae, causes the cigar end of banana. Molecular and genetic studies have shown that these species represent a complex pool of discrete lineages of varying degrees of relatedness with unknown levels of gene flow between them. Most isolates are haploid, but some are thought to be amphihaploid interspecific hybrids. Until our understanding of this complex is much improved, it seems most appropriate to add only one new species, for wilt isolates primarily associated with potato and producing dark-resting-mycelium in bundles (currently known as V. albo-atrum Grp2). It is suggested that the following be retained: (i) V. dahliae to include all isolates which produce only microsclerotia, (ii) V. albo-atrum to cover the majority of isolates producing only dark-resting-mycelium (and not in bundles), and (iii) V. nigrescens, V. nubilum, V. tricorpus and V. theobromae for the minor wilt pathogens/saprophytes and the non-wilt pathogen.