Molecular mapping of a locus controlling resistance to Albugo candida in Brassica rapa

White rust, caused by Albugo candida, is an economically important disease of crucifers. Genetic analysis for resistance to race 2 of A. candida in an F-2 population and a set of F-3 families both derived from a cross between Brassica rapa cultivars Per (resistant) and R-500 (susceptible) revealed that resistance is controlled by a dominant allele at a single locus. White rust resistance was associated with leaf pubescence, which also was governed by a dominant allele at a single locus. The resistance locus (ACA1) was mapped by linkage analysis with 144 restriction fragment length polymorphism (RFLP) loci segregating among the F-3 families. The ACA1 locus was mapped to linkage group 4 and was flanked by RFLP marker loci ec2b3a (5.4 centimorgans [cM]) and wg6c1a (5.0 cM). ACA1 was linked to the leaf pubescence locus PUB1 by 13.3 cM. The linked RFLP markers and leaf pubescence may be useful in introgression and map-based cloning of white rust resistance in B. rapa and its related species.