Genetic variation analysis of field isolates of clubroot and their responses to Brassica napus lines containing resistant genes CRb and PbBa8.1 and their combination in homozygous and heterozygous state

Brassica napus is one of the most important oilseed crops in the world and Plasmodiophora brassicae is a serious threat causing yield reduction. CRb and PbBa8.1 resistance genes are known to be highly effective against P. brassicae race 4. Here, we combined two clubroot resistance (CR) genes through marker-assisted selection (MAS) and developed CR homozygous lines. The CR parental and pyramided lines were crossed with genetically male sterile (GMS) plants to generate heterozygous lines. In order to confirm their differences on resistance, the parental and generated lines were inoculated with 9 P. brassicae field isolates in the greenhouse and resistant test also evaluated in different naturally infested fields. The CR pyramiding lines exhibited high levels of resistance to the most isolates than line containing any single resistance gene; comparatively, the homozygous lines demonstrated a higher resistance than the heterozygous. The genetic variation among P. brassicae isolates was analyzed through simple sequence repeat (SSR) marker and significant divergences were recorded among the 9 tested field isolates and the result was consistent with data acquired from the greenhouse and natural field experiments. This study will certainly provide a novel molecular strategy for the breeding of the durable CR B. napus varieties in future breeding programs.