Quantitative resistance affects the speed of frequency increase but not the diversity of the virulence alleles overcoming a major resistance gene to [i]Leptosphaeria maculans[/i] in oilseed rape

International audienceQuantitative resistance mediated by multiple genetic factors has been shown to increase the potential for 29 durability of major resistance genes. This was demonstrated in the [i]Leptosphaeria maculans / Brassica napus[/i] 30 pathosystem in a 5 year recurrent selection field experiment on lines harboring the qualitative resistance 31 gene Rlm6 combined or not with quantitative resistance. The quantitative resistance limited the size of 32 the virulent isolate population. In this study we continued this recurrent selection experiment in the 33 same way to examine whether the pathogen population could adapt and render the major gene ineffec- 34 tive in the longer term. The cultivars Eurol, with a susceptible background, and Darmor, with quantitative 35 resistance, were used. We confirmed that the combination of qualitative and quantitative resistance is an 36 effective approach for controlling the pathogen epidemics over time. This combination did not prevent 37 isolates virulent against the major gene from amplifying in the long term but the quantitative resistance 38 significantly delayed for 5 years the loss of effectiveness of the qualitative resistance and disease severity 39 was maintained at a low level on the genotype with both types of resistance after the fungus population 40 had adapted to the major gene. We also showed that diversity at the AvrLm6 virulence allele was com- 41 parable in isolates recovered after the recurrent selection on lines carrying either the major gene alone 42 or in combination with quantitative resistance: a single repeat-induced point mutation and deletion 43 events were observed in both situations. Breeding varieties which combine qualitative and quantitative 44 resistance can effectively contribute to disease control by increasing the potential for durability of major 45 resistance gene