Durability of plant major resistance genes to pathogens depends on the genetic background, experimental evidence and consequences for breeding strategies

The breakdown of plant resistance by pathogen populations is a limit to the genetic control of crop disease. Polygenic resistance is postulated as a durable alternative to defeated major resistance genes. Here, we tested this postulate in the pepper-Potato virus Y interaction. The virus was selected for virulence towards monogenic and polygenic host resistance, using serial inoculations in laboratory and in natural epidemic conditions. The frequency of resistance breakdown and the genetic changes in the virus avirulence gene were analysed. The monogenic resistance provided by the pvr2(3) gene was defeated at high frequency when introgressed in a susceptible genetic background whereas it was not when combined to partial resistance quantitative trait loci. The suppression of emergence of virulent mutants because of the genetic background resulted both from a differential selection effect and the necessity for the virus to generate multiple mutations. The virus adaptation to the polygenic resistance required a step-by-step selection with a primary selection for virulence towards the major gene, followed by selection for adaptation to the genetic background. Polygenic resistance proved more durable than monogenic resistance, but breeding strategies giving priority to major resistance factors may jeopardize the progress in durability expected from polygenic resistance. New Phytologist (2009) 183: 190-199doi: 10.1111/j.1469-8137.2009.02827.x.