Breeding Strategies for Improving Plant Resistance to Diseases

Abstract

Durable disease resistance is an important aim in each breeding program. Genetically, two basic patterns of resistance are available: qualitative (race-specific, vertical) and quantitative (race-non-specific, horizontal) resistances. Classical breeding methods are recurrent backcrossing (BC) for introducing single (major) genes, recurrent selection for improving the level of quantitative resistances and multi-stage selection for combining resistances and agronomic traits during cultivar development. Molecular markers allow efficient introduction of qualitative resistances into elite material and to analyze quantitative resistances. During marker-assisted backcrossing (MABC), a major gene can be precisely targeted, the genome of the recurrent parent can be recovered fast, and linkage drag can be reduced. By marker-assisted selection (MAS), major genes or quantitative trait loci (QTL) can be pyramided. Genomic selection (GS) will allow selecting for multiple traits directly in the genome by chip-based, high-throughput genotyping platforms. To achieve a higher durability, populations of biotrophic pathogens (e.g. powdery mildews, rusts) should be regularly monitored for their virulence frequencies and virulence combinations. Strategies for enhancing durability of qualitative resistances aim to increase host diversity or host complexity. Quantitative resistances generally have a higher durability but might be prone to gradual loss (erosion) in the long term. Limits of resistance selection are given by several biological and economic constraints. Broad-spectrum resistance genes and GS might open new avenues to a rational, knowledge-based selection. Resistance breeding will remain a top priority given the challenges of a growing world population in a changing climate.