Inheritance of resistance to the newly identified Plasmodiophora brassicae pathotypes in Brassica napus L.

Clubroot disease caused by Plasmodiophora brassicae Woronin is one of the most devastating diseases of canola (Brassica napus L.). Pathotype 3H has been the most prevalent and virulent pathotype in western Canada. Many resistant cultivars have been developed in Canada in recent years. However, large-scale cultivation of these cultivars resulted in the identification of several new virulent pathotypes including 3A and 5X that overcame this resistance. We studied two doubled haploid (DH) populations, developed by crossing a canola line carrying resistance to 3H, 3A, and 5X to two canola lines carrying resistance to 3H, for resistance to these three pathotypes. Furthermore, two clubroot-resistant populations derived from the first and second cycles of breeding were also investigated for resistance to 10 pathotypes to understand their inheritance. A 3R:1S segregation for resistance to 3H confirmed the involvement of two independent major gene loci in these DH populations. In contrast, a 1R:7S and 1R:15S segregation, respectively, for resistance to 3A and 5X indicated that more than two gene loci are involved in the control of resistance to these pathotypes. A significant positive correlation for resistance to 3A and 5X indicated that some common genes are involved in the control of resistance to both pathotypes. About 4% of the DH lines were resistant to all three pathotypes, suggesting that pyramiding of multiple resistance genes in canola is possible. Overall, the resistance observed in the population derived from the first cycle of breeding remained effective in the population derived from the second cycle of breeding.