Abstract
Verticillium longisporum is a soil-borne vascular pathogen of oilseed rape and other Brassica crops. The limited availability of chemical control measures against vascular pathogens calls for the exploration of control alternatives. Cross-protection may be conveyed by the use of non-pathogenic or non-aggressive isolates of potential pathogens as biocontrol agents. V. longisporum consists of three lineages that originated from three independent hybridization events of four haploid Verticillium parents. Previous pathogenicity tests in the greenhouse have shown that lineage A1/D2 is non-pathogenic or non-aggressive on different Brassica and non- Brassica hosts. Thus, the cross-protection potential of the A1/D2 lineage against an aggressive V. longisporum isolate in oilseed rape was tested. With root-dip inoculation, A1/D2 reduced disease symptoms when applied before or at the same time as the aggressive isolate. The induction of salicylic acid, a signal known to play a role in basal and cultivar-related resistance, was not involved in the biocontrol mechanism. The most practical and best feasible method of application as a seed coat failed to confirm the biocontrol effect of A1/D2 observed with root-dip inoculation. Confocal microscopy analysis revealed that seed coating led to insufficient A1/D2 hyphal establishment on the roots compared to root-dip inoculation, which may explain the lack of a biocontrol effect after seed coating and illustrates the importance of the application method for efficacy of a cross-protective biocontrol agent.