Analyses of virulence of European isolates of clubroot (Plasmodiophora brassicae Wor.) and mapping of resistance genes in rapeseed (Brassica napus L.)
Clubroot caused by the obligate biotrophic protist Plasmodiophora brassicae is a serious soilborne disease of cruciferous crops including oilseed rape (Brassica napus L.). Physiological specialisation of pathogen populations causes differences in pathogenicity, rendering breeding for resistance difficult. Therefore, it is important to get more detailed information on the virulence of P. brassicae in Europe. Samples of infected plants were collected all over the main European oilseed rape growing regions and forty-eight isolates were characterised under greenhouse conditions by artificial inoculation on the European Clubroot Differential (ECD) series and the differential set of Somé et al. (1996) followed by optical rating of disease symptoms. In total, 33 different ECD triplet codes were detected of which classifications ‘16/14/31’, ‘16/31/31’ and ‘17/31/31’ were most common. Based on results obtained on the differentials of Somé et al. (1996) P1 is the prevalent pathotype on oilseed rape fields in the maritime region of Northern Europe whereas P3 was most frequently detected in the continental part of Europe. As breeding for resistance is the most powerful tool to control clubroot, broadening of the genetic basis of resistance in oilseed rape is needed. Therefore, clubroot resistances derived from two rutabaga (Brassica napus var. napobrassica) varieties, i.e., ‘Invitation’ and ‘Wilhelmsburger’, were genetically mapped in doubled haploid (DH) populations of crosses to the susceptible oilseed rape (Brassica napus L.) cultivar ‘Ladoga’. The DH populations were analysed for resistance against two P. brassicae isolates showing different virulence patterns in the greenhouse. The segregation ratios indicated the effectiveness of one, two and three resistance genes, respectively, conferring resistance in these DH populations depending on the P. brassicae isolate used. Studies on F1 plants give hint to dominant resistance genes in both donor lines located on chromosomes A03, A05 and A08.