Abstract
SummaryNear‐isogenicBrassica napuslines carrying/lacking resistance geneRlm6were used to investigate the effects of temperature and leaf wetness duration on phenotypic expression ofRlm6‐mediated resistance.Leaves were inoculated with ascospores or conidia ofLeptosphaeria maculanscarrying the effector geneAvrLm6. Incubation period to the onset of lesion development, number of lesions and lesion diameter were assessed. Symptomless growth ofL. maculansfrom leaf lesions to stems was investigated using a green fluorescent protein (GFP) expressing isolate carryingAvrLm6.L. maculansproduced large grey lesions on Darmor (lackingRlm6) at 5–25 °C and DarmorMX (carryingRlm6) at 25°C, but small dark spots and ‘green islands’ on DarmorMX at 5–20°C. With increasing temperature/wetness duration, numbers of lesions/spots generally increased. GFP‐expressingL. maculansgrew from leaf lesions down leaf petioles to stems on DarmorMX at 25°C but not at 15°C.We conclude that temperature and leaf wetness duration affect the phenotypic expression ofRlm6‐mediated resistance in leaves and subsequentL. maculansspread down petioles to produce stem cankers.
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