Introgressing Multiple Disease Resistance Alleles into Elite Apple Cultivars By DNA-informed Breeding

Plant diseases cause substantial economic losses for the worldwide apple industry. For example, powdery mildew is prevalent and damaging in the major U.S. production regions and blue mold is destructive during fruit storage. Chemical controls are costly and potentially damaging to the environment and human health. Planting disease resistant cultivars is a promising way to mitigate plant diseases. However, single sources of genetic resistance might limit the choice of commercial cultivation conditions. Cultivars with multiple disease resistance could be grown in diverse growing region of the world. Although many disease resistant apple cultivars have been released, none are widely planted due to less-than-ideal genetic potential for fruit quality. Introgressing multiple disease resistance alleles into single elite cultivars is time-consuming and inefficient owing to the extended juvenility of the crop and the need to purge undefined poor fruit-quality alleles over multiple generations, and the difficult evaluation of multiple disease susceptibilities phenotypically. DNA-informed breeding employed by collaborating breeders in the RosBREED project could make this introgression more efficient. The objective of this study is to enable breeders to select cultivars with durable disease resistance and superior fruit quality by 1) developing DNA tests for powdery mildew and blue mold resistance and 2) developing DNA-informed introgression strategies for multiple resistances and demonstrating them in the Washington State University Apple Breeding Program. Targeting previously discovered breeding-relevant trait loci, trait-predictive DNA tests to detect Pl-w (from the cultivated crab apple ‘White Angel’, with wild species ancestry of Malus sargentii and M. sieboldii) and Pe3 (from wild species M. sieversii PI613981) resistance alleles were developed. Two segregating populations consisting of 100 and 230 individuals from ‘Sunrise Magic’ x ‘White Angel’ and ‘Royal Gala’ x PI613981, respectively, were utilized to establish associations between DNA markers and disease susceptibility. DNA test Plw-SSR targeting Pl-w locus could accurately detect offspring with powdery mildew resistance. DNA test Pe3s-SSR can identify seedlings with about 60% smaller lesion diameter compared to the susceptible sibling seedlings. Concurrently, introgression strategies, e.g., marker-assisted foreground selection for known fruit quality and disease resistance alleles and marker-assisted background selection against presumably undesirable wild alleles from donor parents, are under investigation. Further alleles conferring powdery mildew resistance from other sources, e.g., M. robusta and M. zumi, could be incorporated into elite cultivars through the application of introgression strategies developed in this project.