Genetics of Brassica rapa (syn. campestris). 2. Multiple disease resistance to three fungal pathogens: Peronospora parasitica, Albugo candida and Leptosphaeria maculans.

Although the genetic basis of multiple disease resistance (MDR) is poorly understood, it is of great value for understanding the evolution of disease resistance in natural plant populations and for increasing crop yields in agriculture. In Brassica rapa, we studied genetic correlations among levels of disease resistance to three fungal pathogens: Peronospora parasitica, Albugo candida and Leptosphaeria maculans. A large, replicated quantitative genetics experiment used artificial selection on resistance to individual pathogens, and examined correlated responses to selection for resistance to other, unselected pathogens. Data from 9518 plants, each measured simultaneously for resistance to three fungal pathogens, showed heritable genetic variation for resistance to each pathogen and a positive genetic correlation between resistance to P. parasitica and L. maculans. This indicates that some resistance genes provide defence against fundamental characteristics common to two taxonomic orders of fungal pathogens. Conceivably, such MDR could contribute to a durable defence that might not be easily circumvented by rapidly evolving fungal pathogens.