Host Specialization among Vegetative Compatibility Groups of Verticillium dahliae in Relation to Verticillium longisporum

A collection of 24 isolates of Verticillium dahliae and 10 isolates of Verticillium longisporum originating from nine different host plants and from several geographic regions was tested for host specificity on 11 economically important crops such as potato, tomato, strawberry, linseed, three legumes and four Brassica species. In order to reveal host specificity the potential of each isolate to induce disease and affect plant yield was recorded for all isolate–host combinations. The collected data were statistically processed by means of a cluster analysis. As a result, the host range of individual isolates was found to be more dependent on the vegetative compatibility group (VCG) of the isolate than on its original host plant provenance. Twenty-two out of 24 V. dahliae isolates belonged to either VCG 2B or 4B. VCG 2B isolates showed specificity for legumes, strawberry, potato and linseed, whereas VCG 4B was specifically virulent on potato, strawberry and linseed. Subgroups within VCG 2B and 4B almost lacking any host preference were designated 2B* and 4B*. Three isolates from VCG 2B*, however, severely attacked tomato which is a host outside the authentic host range of VCG 2B. The pathogenicity of V. longisporum isolates was restricted to cruciferous hosts. Conversely, cruciferous plants were not affected by isolates from VCGs 2B and 4B of V. dahliae. This lack of crossinfectivity of certain subpopulations of V. dahliae and of V. longisporum may be useful in the management of this soil-borne wilt disease.