RAPD markers linked to locus controlling resistance for race 4 of the black rot causative agent, Xanthmonas campestris pv. campestris (Pamm.) Dow. inBrassica rapa L.

Association between the RAPD markers and the resistance to race 4 of the bl ack rot causative agent was studied in Brassica rapa L. Experiments were ca rried out using doubled haploid lines, obtained via crosses between the rac e 4-susceptible fodder turnip and resistant pak-choi, and the F-2 progeny o f the crosses between the doubled haploid lines with contrasting resistance . The WE22(980) RAPD marker inherited from the pak-choi and associated with the clubroot susceptibility was also linked to the locus responsible for t he resistance to race 4 of Xanthomonas campestris pv. campestris. The two o ther RAPD markers were linked to susceptibility to black rot. Simultaneous association of the same DNA markers with the resistance/susceptibility to t wo different obligate pathogens favored the hypothesis on cluster organizat ion of the resistance genes in plants. The markers described can be used in plant breeding and in further investigation of the genetic bases of resist ance in plants.