The molecular taxonomy of plant pathogenic Verticillium species

Molecular biological techniques are now being used to study many aspects of plant pathogens of all types and my own group at Horticulture Research International is, in collaboration with others, currently involved in such studies on various fungi, phytoplasmas, plant viruses and mites. For some time our major interest has been in the Verticillium wilt fungi. Conventional mycology suggested that V. alboatrum and V. dahliae (haploid isolates on ly) were homogeneous spec ies with only a limited degree of host specialization but that the diploid isolates of V. dahliae, largely from Brassica spp., formed a clearly distinct group. Molecular studies, from our work and elsewhere, have revealed several surprises. For example, some isolates of 'V. alboatrum' are clearly a very distinct species whilst so-called 'diploid isolates of V. dahliae' appear more closely related to V. alboatrum than to haploid isolates of V. dahliae. For practical purposes in the UK, the most interesting findings are that haploid V. dahliae clearly divides into two groups of differing average pathogenicity and that it is probably a recently introduced pathogen, still spreading geographically. This paper will describe our work in more detail and put it into context with that of some other groups and some general considerations concerning molecular taxonomy.