2019 ASHS Annual Conference

The long generation time of apple (Malus pumila Mill.) has made the development of new cultivars with pyramided disease resistance alleles to multiple diseases difficult to achieve. Furthermore, because the presence of a single resistance allele can phenotypically mask the presence of additional alleles, pyramiding resistance alleles by traditional phenotyping methods can be challenging or impossible. Ectopic expression of a FRUITFUL homolog (BpMADS4) from silver birch (Betula pendula Roth) reduces the juvenility of apple from 3-7 years to 3-8 months. Since the BpMADS4 transgene behaves genetically as a single dominant allele at a heterozygous locus, it segregates 1:1 in subsequent crosses such that 50% of the resulting population will carry the BpMADS4 transgene for reduced juvenility, and 50% will be non-transgenic with normal growth and juvenility. Individuals carrying the BpMADS4 transgene are initially selected to facilitate accelerated breeding during the pyramiding of desirable alleles at multiple trait loci. After achieving the desired allele accumulation, non-transgenic individuals without the BpMADS4 transgene but retaining the desired trait alleles are selected. Our goal in RosBREED was to pyramid both scab (Venturia inaequalis) and fire blight (Erwinia amylovora) resistance alleles in apple breeding parents. DNA-informed breeding methods, rather than traditional phenotyping methods, were used to identify individuals with multiple alleles for disease resistance and elite fruit quality. Disease resistance alleles and the BpMADS4 transgene were actively selected using locus-specific DNA tests in Marker-Assisted Seedling Selection (MASS). Following MASS of progeny for the targeted alleles, seedlings were evaluated by Marker-Assisted Parent Selection (MAPS) using a panel of DNA tests for important fruit quality traits developed in RosBREED. Individuals to advance to the next generation were identified based on their overall allele composition in MAPS rather than by the presence of specific fruit quality alleles. Similar methodology was used to introgress resistance to post-harvest apple blue mold from M. sieversii PI613981 into apple breeding parents with other pyramided disease resistance alleles. To achieve efficient introgression of the blue mold QTL, however, MASS individuals were genotyped on apple 20K Illumina SNP array to identify favorable recombination events near the blue mold QTL and quantify undesirable DNA segments from PI613981. The development of breeding parents containing pyramided alleles for resistance to apple scab and fire blight along with many desirable fruit quality traits, will allow breeders to obtain new cultivars with a desired suite of resistance alleles from one cross rather than from multiple crosses over many generations.