Pathogenicity of leptosphaeria maculans isolates obtained from brassica napus (oilseed rape) cultivars with the rlm7 resistance gene

Isolates obtained from winter oilseed rape cultivars with the resistance gene Rlm7 were examined for their pathogenicity by inoculation onto cotyledons and true leaves of the susceptible cultivar Drakkar (no R gene) and cultivars with the Rlm7 gene (Excel, Roxet, Hearty and line 01-23-2-1). After assessment of lesions, cotyledons and true leaves were detached 17 and 21 dpi, respectively, and incubated in darkness under high humidity to assess pycnidial development and conidial production. All the isolates tested produced typical large/grey lesions (susceptible phenotype) on both cotyledons and true leaves of Drakkar; large numbers of pycnidia with conidial masses were produced in and outside the lesions on cotyledons after 5 days of incubation and on true leaves after 3 days of incubation. Small lesions surrounded by dark margins (resistant phenotype), with no difference between isolates in lesion area, were produced on cotyledons of the four Rlm7 cultivars. There was no difference in lesion area between the isolates tested on true leaves of Excel, Hearty and 01-23-2-1 but there was difference between isolates on Roxet. Most of the isolates produced small numbers of immature pycnidia on the lesions on cotyledons after 5 days of incubation and on true leaves after 3 days of incubation of the four cultivars with the Rlm7 gene. However, these pycnidia were not able to produce conidia. With increased incubation period and the senescence of cotyledons and true leaves, mature pyncidia developed and produced conidia outside the lesions.