Effects of temperature on R1m6-mediated resistance to Leptosphaeria maculans in Brassica napus

Near-isogenic lines of Brassica napus lines with/without the resistance gene Rlm6 were used to investigate the effects of temperature on Rlm6-mediated resistance to Leptosphaeria maculans. Leaves were inoculated with ascospores of L. maculans carrying the corresponding avirulence gene AvrLm6. Inoculated plants were incubated at 15°C or 25°C. DarmorMX (with Rlm6) was resistant to L. maculans at 15°C but susceptible at 25°C, and Darmor (without Rlm6) was susceptible at both temperatures. On Darmor, large grey leaf lesions developed at both 15 and 25°C. On DarmorMX, small dark necrotic spots were produced at 15°C but large grey lesions were produced at 25°C. The incubation period of L. maculans (from inoculation to the appearance of first lesion) was longer on DarmorMX than on Darmor. The infection efficiency (number of lesions resulting from inoculation with 100 ascospores) was greater on Darmor than on DarmorMX at 15 and 25°C. Further characterisation of the Rlm6-mediated resistance showed that the resistance was effective at 25°C if inoculated plants were pre-incubated at 15°C for 5 days before moving them to 25°C, but not if inoculated plants were pre-incubated at 15°C for only 2 days before moving them to 25°C. We conclude that temperature affects Rlm6-mediated resistance to L. maculans in B. napus leaves.