| Summary: | <i>Leptosphaeria maculans</i> is one of the major pathogens of oilseed rape (<i>B. napus</i>). It causes blackleg disease, which accounts for significant yield losses worldwide. Using cultivars that harbor major resistance (<i>R</i>) genes is one of the most effective control methods. However, the efficacy of major <i>R</i> genes is related to the frequency of the corresponding avirulence (<i>Avr</i>) genes in a <i>L. maculans</i> population. In this paper, we report the <i>Avr</i> profiles of <i>L. maculans</i> populations and the ratio of its mating types in Northern and Central regions of Germany. Eleven <i>Avr</i> genes in five-hundred and seventy-four isolates were characterized either by applying cotyledon tests on a <i>B. napus</i> differential set or by amplifying avirulence gene-specific PCR markers. Fifty-two races were determined, among which the most dominant race was <i>Avrlm6, -7, -11, AvrlepR1, -R2</i>. Results showed that the resistance gene <i>Rlm2</i> is 100% ineffective, some other major <i>R</i> genes such as <i>Rlm1, Rlm3</i>, <i>Rlm4</i> and <i>LepR3</i> are partially effective (with corresponding <i>Avr</i> frequencies ≤ 42%), while <i>LepR1</i>, <i>LepR2</i>, <i>Rlm6</i>, <i>Rlm11</i> and <i>Rlm7</i> can still provide relatively effective resistance in the German fields investigated (with corresponding <i>Avr</i> frequencies of 63–100%). Sexual reproduction is a factor that enhances the potential of <i>L. maculans</i> to evolve under selection pressure. Mating types of the <i>L. maculans</i> populations did not deviate from the ratio of 1:1 in the examined regions, indicating that sexual reproduction and ascospores play central roles in the <i>L. maculans</i> lifecycle. Overall, this study provides an important dataset for the establishment of a strategic plan to preserve the efficacies of major <i>R</i> genes in Germany by applying cultivar rotations of oilseed rape.
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