Phylogenetic analyses of phytopathogenic isolates of Verticillium spp.
To better understand the genetic relationships between Verticillium dahliae isolates from lettuce and other phytopathogenic Verticillium spp. isolates from various hosts and geographic locations, the complete intergenic spacer (IGS) region of the nuclear ribosomal RNA gene (rDNA) and the β-tubulin gene were amplified and sequenced. The sequences of the complete IGS region and the β-tubulin gene were used alone and in combination to infer genetic relationships among different isolates of Verticillium with the maximum-likelihood distance method. Phylogenetic analyses set sequences into four distinct groups comprising isolates of V. albo-atrum, V. tricorpus, and V. dahliae from cruciferous and noncruciferous hosts. Within the four Verticillium groups, isolates of V. dahliae from cruciferous hosts displayed the closest affinity to V. dahliae from noncruciferous hosts. Isolates of V. dahliae from noncruciferous hosts could be further divided into four subgroups based on sequence similarities within the IGS region. Cross-pathogenicity tests demonstrated that most Verticillium isolates were as virulent on other hosts as on their hosts of origin. A phenogram based on the cross pathogenicity of individual isolates resembled those derived from the IGS and β-tubulin sequence comparisons. On the basis of the data presented, the potential origin of some isolates of V. dahliae pathogenic on lettuce is proposed.