Efficacy of major resistance genes against Leptosphaeria maculans in oilseed rape and influence of temperature on the plant-pathogen interaction

Stem canker caused by Leptosphaeria maculans is an important disease in oilseed rape (Brassica napus L.) and causes significant yield losses. The cultivation of resistant cultivars is a reliable tool for protecting the crop against pathogen damage. In controlled environment experiments, the efficacy of major resistance genes against L. maculans was tested under varying temperatures for cotyledons and stems. Therefore, the resistant cultivars Caiman with Rlm7 resistance and Uluru with LepR3 resistance, as well as Lirabon as susceptible control, were used. For each resistant cultivar an avirulent and a virulent L. maculans isolate were selected. Cotyledon resistance was tested with a spore suspension, whereas adult resistance was tested at the stem base by inoculation with a mycelium plug. The plant‐pathogen interactions were examined at different temperature regimes. Incompatible interactions found on cotyledons of Uluru turned to be compatible, whereas only an increase of L. maculans DNA was found for cotyledons of Caiman at higher temperatures (≥27°C). Major gene resistance could actively reduce disease severity in stem tissue. Especially, Caiman was strongly dependent on its Rlm7 resistance gene, whereas resistance of Uluru relied more on quantitative resistance. High temperature treatment did not change incompatibility into compatibility at stem bases.