Quantitative resistance associated with a very efficient major gene in a single Brassica napus genotype increases the potential durability of the resistance to Leptosphaeria maculans

摘要

The durability of resistance is an important question in plant disease resistance breeding Resistance to Leptosphaeria maculans conditioned by Rlm6 (MX), which was introgressed from Brassica juncea to a susceptible B. napus genotype is very high but is not durable. In order to determine the effect of the polygenic resistance of oilseed rape on the potential durability of this major gene, 2 oilseed rape genotypes (Eurol and Darmor) with differences in quantitative resistance at the adult stage were assessed in pairs of near-isogenic lines differing for Rlm6. The lines into which the major gene was introgressed (EurolMX and DarmorMX) were highly resistant when evaluated using the standard cotyledon stage test and cotyledon or leaf infections were rarely observed in the field. Cotyledons and leaves of Darmor (polygenic resistance only) displayed high levels of infection (very susceptible) but this genotype displayed pardal resistarce at the adult plant stage. One field experiment was conducted for 4 years following a protocol developed to increase the selection pressure of L. maculans populations on the resistance of B. napus genotypes. Each year, 4 separate fields were established in which fungal populations were selected recurrently on residue of 1 of the 4 B. napus genotypes (Eurol, EurolMX, Darmor, DarmorMX), which was used as primary inoculum in the next year. MX resistance broke down in the fourth year of the study in the Eurol genetic background when the primary inoculum was recurrently selected on EurolMX. The resistance of DarmorMX, which carries both types of resistance, remained effective in the fourth year of the experiment when the primary inoculum was recurrently selected on the residue of DarmorMX. These results suggest a slower adaptation of the virulent isolates on this line and therefore an effect of the polygenic quantitative resistance on the durability of the resistance.