Detection of Leptosphaeria maculans races on winter oilseed rape in different geographic regions of Germany and efficacy of monogenic resistance genes under varying temperatures

Blackleg disease, caused by Leptosphaeria maculans is one of the most important fungal diseases in oilseed rape production word-wide (Fitt et al. 2006). Genetic resistance is an important tool to control this disease. Seedling resistance is conferred by single major genes. Due to its sexual propagation, L. maculans isolates evolve rapidly from avirulent to virulent strains on cultivars harbouring major resistance genes. Therefore, resistance of oilseed rape against L. maculans conferred only by major resistance genes was often overcome and led to severe yield losses in the past in France and Australia (Rouxel et al. 2003; Sprague et al. 2006). Therefore, we cultivated two oilseed rape (OSR) cultivars in 4 different geographical regions in northern Germany in the growing seasons 2011/12 and 2012/13: i) one cultivar harboring no known major gene against L. maculans (Lirabon) and ii) one resistant cultivar (Exocet), harboring the major gene Rlm7. In autumn and spring we collected true leaves with typical Phoma lesions to gain isolates of L. maculans. Isolates obtained from leaves of Lirabon were considered to represent the whole range of virulent isolates in a region. Single spore isolates were tested on a French differential set consisting of 7 OSR genotypes with known major resistance genes for the presence of the avirulence genes Avr1, 2, 3, 4, 7 and 9 in a cotyledon inoculation test. Thereby, the frequency of virulent isolates in a region was determined. Isolates gained from Exocet were considered to represent the frequency of Rlm7 resistance breaking isolates, which was tested in the cotyledon inoculation test with a Rlm7 harboring cultivar (Caiman). The frequency of virulent isolates on Rlm1, 2, 3, 4 and 9 was very high with over 80%. The frequency of virulent isolates on Rlm7 was very low (< 5%). We assume that choice of cultivars with different compliment of resistance genes leads to a different spectrum of virulent isolates per region. Furthermore we tested the efficacy of major resistance genes against L. maculans under varying temperatures for cotyledons and stems in controlled-environment experiments. 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 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 actively reduced 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.