Virulence/avirulence patterns among Leptosphaeria maculans isolates determines expression of resistance, senescence and yellowing in cotyledons of Brassica napus

Blackleg, (stem canker, Leptosphaeria maculans), is the most economically important disease on oilseed rape (Brassica napus). Studies were undertaken to determine the effect of three different L. maculans isolates with different virulence/avirulence patterns on the expression of qualitative resistance and senescence on cotyledons of 11 B. napus genotypes. There were significant differences between genotypes (P < 0.01), isolates (P < 0.01) and a significant genotype by isolate interaction (P < 0.01) in terms of disease index and lesion diameter. Overall, B. napus LSF0530 was more resistant than Barrel > Scotia > Amber x Commanche DH Line. Westar, Drummonds Purple Top, and Parkside were the most susceptible. L. maculans isolates D5 and D13 were the most pathogenic, showing the greatest disease index and lesion diameters. Genotypes such as LSF0530, Scotia, Duell and Barrel responded with a distinct hypersensitive response, whereas Parkside, Drummonds Purple Top, Amber x Commanche DH Line and Westar all showed a distinct yellow halo around the lesions, indicative of cotyledon senescence likely involving programmed cell death (PCD). However, expression of the distinct yellow halo symptom was not observed in all B. napus genotypes infected by one specific L. maculans isolate nor in one specific B. napus genotype challenged with all the isolates. We believe that this is the first report, i), showing the involvement of a distinct yellow halo (senescence/PCD) associated with both ‘typical’ lesions and with an expression of HR in cotyledons of winter-type B. napus genotypes challenged with L. maculans isolates and, ii), that expression of the distinct yellow halo is clearly dependent on the interaction between particular isolate virulence(s) x particular winter-type B. napus genotype combinations.