Characterisation and pathogenicity of Verticillium dahliae isolates associated with Verticillium wilt of tomato in the Limpopo Province of South Africa

In the Limpopo Province of South Africa, the sustainable production of tomatoes is threatened by a soilborne disease known as Verticillium wilt. Limited information is available regarding the Verticillium species involved, their genetic diversity and aggressiveness. Therefore, a pathogen survey was conducted in the three major tomato production regions in Limpopo (six fields in the Lowveld and two fields each in the Highveld and Soutpansberg). Verticillium dahliae was identified as the only Verticillium species associated with Verticillium wilt of tomatoes. Conventional vegetative compatibility group (VCG) testing identified 38 isolates as VCG 4B and two isolates as VCG 2B. Pathotype-genotype specific PCR analysis indicated that all the isolates belonged to the non-defoliating pathotype, specifically genotype C. These characteristics allowed for the classification of almost all the isolates from the Lowveld, Highveld and Soutpansberg regions into clonal lineage 4B (38 isolates). The exception was for two isolates from the Lowveld that belonged to lineage 2B824. Based on a race-specific PCR analysis, all 40 isolates belonged to race 2. Sequencing of the race-specific PCR amplicon revealed the presence of two haplotypes namely I and II. Evaluation of seven inoculation methods (agar plug, millet, node-inoculation, root-dip, sand-bran, soil-drench and toothpick methods) using three isolates showed that the root-dip method was the only method that consistently identified all the isolates as pathogenic based on disease severity as well as an increase in plant height. Assessment of the aggressiveness of the 40 characterised V. dahliae isolates showed that the isolates varied in aggressiveness to the cv. Floradade.