Temperature and leaf wetness duration affect phenotypic expression of Rlm6-mediated resistance to Leptosphaeria maculans in Brassica napus

Near-isogenic Brassica napus lines carrying/lacking resistance gene Rlm6 were used to investigate the effects of temperature and leaf wetness duration on phenotypic expression of Rlm6-mediated resistance. Leaves were inoculated with ascospores or conidia of Leptosphaeria maculans carrying the effector gene AvrLm6. Incubation period to the onset of lesion development, number of lesions and lesion diameter were assessed. Symptomless growth of L. maculans from leaf lesions to stems was investigated using a green fluorescent protein (GFP) expressing isolate carrying AvrLm6. L. maculans produced large grey lesions on Darmor (lacking Rlm6) at 5-25 degrees C and DarmorMX (carrying Rlm6) at 25 degrees C, but small dark spots and 'green islands' on DarmorMX at 5-20 degrees C. With increasing temperature/wetness duration, numbers of lesions/spots generally increased. GFP-expressing L. maculans grew from leaf lesions down leaf petioles to stems on DarmorMX at 25 degrees C but not at 15 degrees C. We conclude that temperature and leaf wetness duration affect the phenotypic expression of Rlm6-mediated resistance in leaves and subsequent L. maculans spread down petioles to produce stem cankers.