Breeding Elite Lines of Apple Carrying Pyramided Homozygous Resistance Genes Against Apple Scab and Resistance Against Powdery Mildew and Fire Blight

Abstract

The development of high-quality cultivars, with durable disease resistance, is a major objective of apple breeding. The selection procedures of modern breeding programs no longer rely exclusively on phenotypic criteria but include marker-assisted breeding (MAB). Currently, molecular markers linked to several resistance genes and quantitative trait loci (QTLs) are available. In this study, we focused on markers available for resistance breeding against the major diseases scab (Venturia inaequalis), powdery mildew (Podosphaera leucotricha), and fire blight (Erwinia amylovora). One approach proposed to achieve durable resistance is the pyramiding of functionally different resistance genes against the same pathogen. This approach can be complemented with the incorporation of resistance genes against other pathogens. The resulting resistant apple cultivars would contribute considerably to low-input, sustainable, fruit production. Furthermore, apple cultivars can be developed carrying homozygous allele sets of specific resistance genes, and these genotypes can be used as parents for further crosses. Due to the ensured inheritance of the resistance genes to the progeny, MAB for these genes will become superfluous. In this study, we developed elite apple plants which are homozygous for three different scab resistance genes, Rvi6, Rvi2, and Rvi4. Furthermore, these apple selections tested positive for a resistance gene against powdery mildew (Pl1 or Pl2), and the FBF7 QTL from ‘Fiesta’ for enhanced fire blight resistance. Selected progeny plants were tested for their fire blight resistance after artificial shoot inoculation and evaluated for tree and fruit characteristics.