Quantitative resistance may confer a general infection defense mechanism that allows for protection from all races of a particular pathogen. The objective of this study was to evaluate the impact of three cycles of mass selection for resistance to A. candida, Race 2, on resistance to A. candida, Race 7. After three cycles of selection for resistance to A. candida, Race 2, cycle 3 was significantly more resistant than the original population to both Race 2 and Race 7. These data strongly suggest that B. rapa utilizes both RNR and RSR types of resistance.
Races are most easily determined by their different host ranges.
A. candida, Race 7, is the natural pathogen of B. rapa. This phenomenon of minor genes creating a major aggregate phenotypic effect is useful for efficient protection of a crop from all races of a pathogen. Plant populations that have undergone selection for RNR should exhibit a significant increase in resistance to all races of a particular pathogen. Because no major resistance genes existed in the original population, the authors suggested that this resistance is quantitatively inherited. Significant increases in resistance to both races occurred over two cycles, indicating this resistance may be race nonspecific.