Breeding for durable resistance in crops: defeated loci may act as Trojan horses compromising the effectiveness of major resistance genes

Resistance breeding offers invaluable perspectives for environment-friendly crop protection, but its success may be limited by the breakdown of plant resistance by pathogen strains. This threat is particularly acute for perennial crops, which may be cultivated for several decades. With the increasing use of new varieties carrying multiple major resistance loci, grapevine (Vitisspp.) represents a distinctive model to investigate the broad agreement that combining several resistance genes (pyramiding) enhances both resistance efficacy and durability. To this end, grapevine progenies segregating for four resistance loci againstPlasmopara viticola(Rpvs) were used to evaluate the efficiency of single and pyramided major loci when confronted to naive andRpv-breaking pathogen strains. In the context of polygenic resistance, both undefeated and defeatedRpvsprovided significant quantitative effects. However, interactions between pyramidedRpvswere either beneficial, neutral or detrimental to the level of resistance, depending on the loci combination and pathogen strain. In particular, the fact that the presence of defeated resistance loci may compromise the resistance provided by functional major loci has important implications for crops resistance breeding. Thorough phenotypic investigations of pyramiding breeding schemes emerge as a critical step for the effective and durable management of genetic resistances and plant diseases.