Development of monogenic lines for resistance to Albugo candida from a Canadian Brassica napus cultivar

Resistance to Albugo candida race 7 in the Canadian Brassica napus cultivar Regent is conditioned by three dominant genes, designated Ac7(1), Ac7(2), and Ac7(3). Gene Ac7(3) is present in a heterozygous condition. Homozygous resistant BC1F3 lines carrying Ac7(1) or Ac7(2) were developed from a BC1F1 family in which segregation for white rust resistance fitted a 3 resistant (R):1 susceptible (S) ratio. To isolate the resistance genes, one BC1F3 line was assumed to have genotype Ac7(1)Ac7(1)ac7(2)ac7(2) and was used as a tester to cross with other selected BC1F3 lines. Progenies from test crosses were self-pollinated and backcrossed to susceptible line 2282-9. Thus, two monogenic lines possessing Ac7(1) or Ac7(2) were developed as F-2 and backcross populations produced from four test crosses segregated to fit 15 R:1 S and 3 R:1 S ratios, respectively, whereas the other four lines were homozygous resistant. The two single-gene lines were used as testers to develop a monogenic line with Ac7(3) from a BC1F1 family that segregated in a 7 R:1 S ratio. These single genes are being incorporated into rapid-cycling B. napus lines susceptible to several pathotypes of A. candida to develop isogenic differential lines. The monogenic lines will be used to study the mechanism(s) of resistance response conditioned by the individual genes. These lines also should facilitate molecular mapping of the loci in B. napus for resistance to A. candida race 7.