The perspectives of polygenic resistance in breeding for durable disease resistance

Polygenic resistance is generally quantitative without clear race specific effects. With the onset of molecular markers technologies, the identification of chromosome regions that are involved in quantitative resistance has become feasible. These regions are designated quantitative trait loci (QTLs). The mapping of `major' QTLs can be independent of environment, season, year or race of the challenging pathogen. However, the detection of minor QTLs may be dependent on the `environment'. As QTLs are defined by the position on the genome and the quantitative effect on resistance, they are not informative about the mechanism of resistance. By comparing QTL with the loci that are involved in race specific resistance the coincidence of these loci may suggest a common mechanism. However, the histological characterisation of the resistance is more informative about the resistance mechanism. Estimations about the durability of polygenic, quantitative resistance are still academic as there is hardly any experience with large-scale usage of quantitative resistance over a longer period. The clearest example of non-durable resistance is race specific monogenic resistance that is associated with a hypersensitive response (HR). Hence, there is a great chance that polygenic resistance that is not associated with HR is more durable. In some pathosystems with a long experience with non-durable race specific HR genes, quantitative resistance offers a good alternative and marker-assisted breeding will facilitate the exploitation of these resistance for commercial purposes.