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Clubroot,caused by Plasmodiophora brassicae Wor.,is one of the most serious diseases affecting Brassica crops production worldwide.Quantitative trait loci(QTLs)for resistance to clubroot disease were studied using a F2:3 populations derived from a cross between clubroot resistant(CR)turnip inbred line SG(Brassica rapa ssp.rapifera)and a clubroot susceptible inbred line of Chinese cabbage BJN(B.rapa ssp.pekinensis).F3 families were inoculated with the isolates of Pb4 and Pb10 and their combination(Pb4+Pb10)under controlled conditions.A total of 4 QTLs,including 1 QTL detected by Pb4,2 QTLs by Pb10,and 1 QTL by Pb4+Pb10,were detected on chromosome A3 and A8 of B.rapa.These QTLs were assigned to two chromosomal regions where two CR genes Crr1 and CRb were identified in the previous studies.One QTL co-localized with the CRb locus was common for resistance to all isolates tested,but contributed different effects to isolate Pb4,Pb10,and Pb4+Pb10.Another QTL mapped in the region of Crr1 showed isolate-specific resistance to Pb10.Additionally,epistatic interaction between CR QTL and non QTL region or between two CR QTL regions with additive effects was detected by isolate Pb4 and Pb4+Pb10.Identification of CR QTLs and genetic effects to clubroot disease will be informative for clubroot resistant breeding in Brassica crops.