Defeated major resistance loci may act as Trojan horses compromising resistance pyramiding in grapevine

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. With the breeding and use of varieties carrying multiple genetic resistances, grapevine (Vitis spp.) represents a distinctive model for perennials to investigate the agreement that pyramiding broadens and enhances the efficacy and durability of resistance. To this end, grapevine progenies segregating for four major resistance loci against Plasmopara viticola (Rpvs) were used to evaluate single and pyramided loci when confronted with naive and Rpv-breaking pathogen strains. In pyramiding, undefeated and defeated Rpvs provided either beneficial, neutral, or detrimental quantitative effects, depending on the loci combination and pathogen strain. In particular, the fact that defeated loci may compromise resistance highlighted important implications for the breeding of perennials. Thorough phenotypic investigations of pyramiding schemes emerge as a critical step for the effective and durable management of genetic resistances.