Multiscale phenotyping and decision strategies in breeding for resistance

Advances in biotechnology have rendered tracking of quantitative trait loci (QTLs) a much easier task, making phenotyping, and not genotyping, the main bottleneck to integrating quantitative host plant resistance into breeding programs. The relevance of phenotyping methods is conditioned by their ability to predict the performance of a genotype at the field scale. Components of resistance represent the keystone hierarchy level between resistance expression in the field (the breeder's scale) and QTLs (the geneticist's scale). We describe approaches for upscaling processes to identify components of resistance that best predict field resistance, and for decision making for selection in breeding programs. We further highlight avenues for future research considering specific processes: disease transmission, defoliation, disease escape, polyetic processes, and interactions between components of resistance.