Differences in the responses of stem tissues of spring-type Brassica napus cultivars with polygenic resistance and single dominant gene-based resistance to inoculation with Leptosphaeria maculans

Six spring-type Brassica napus L. cultivars, either susceptible or with polygenic or monogenic resistance, were inoculated with Leptosphaeria maculans (Desmaz.) Ces. & De Not. (organism causing phoma stem canker in crucifers) to investigate differences in the responses of host stem tissues to the pathogen. At growth stage 1.06, plants were inoculated with pycnidiospores at the junction of the petiole and stem. The pre-penetration and penetration phases were examined along with the histological, ultrastructural, and histochemical responses. The processes of pycnidiospore attachment, germination, and penetration through the stomata of petioles and stems were found to be similar in all cultivars. Specific post-penetration defense reactions identified were lignification, suberisation, and additional cambium formation in the resistant cultivars. In ‘Surpass 400’, which has monogenic resistance, these responses occurred 4–5 d earlier than in polygenically resistant cultivars, and were more intense (preventing hyphal penetration of the additional cambium layer), and resulted in a hypersensitive reaction without pycnidia formation. Our study clearly emphasizes the variatiability in location, timing, and histochemistry of stem responses between compatible and incompatible interactions and will improve our overall understanding of the role and importance of the mechanisms of resistance in spring-type B. napus to L. maculans.