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Apple blue mold causes significant postharvest economic losses worldwide. A blue mold resistance locus, qM-Pe3.1, was previously identified on chromosome 3 of Malus sieversii PI 613981, a wild accession with inferior fruit quality. Introgression of the resistance allele into elite breeding germplasm is difficult and success of introgression and the effect of the PI 613981 genome on fruit quality cannot be phenotypically evaluated until fruiting, which occurs approximately 5 years from seed. In this study, introgression of the qM-Pe3.1 resistance allele was achieved by rapid cycle breeding, utilizing the transgenic line T1190 constitutively expressing the BpMADS4 early-flowering gene. This was supported by DNA-based diagnostic information that enabled marker-assisted selection for blue mold resistance using a locus-specific DNA test developed to detect the qM-Pe3.1 resistance allele in offspring (foreground selection). Of 75 second-generation (['Gala' × PI 613981] × T1190) offspring carrying BpMADS4, 43 also carried the qM-Pe3.1 resistance allele. DNA tests for other trait loci were used to identify other desirable alleles related to fruit quality in progeny and 6874 genome-wide SNP markers (from an apple 20K Illumina® SNP array) were used to identify undesirable genomic segments of PI 613981 (background selection). Three individuals identified with favorable recombination close to qM-Pe3.1 and less than 25% of M. sieversii unimproved genome were selected as best suited for the elimination of unimproved DNA segments in subsequent generations. Our pipeline for introgression of qM-Pe3.1, facilitated by marker-assisted foreground and background selection, successfully advanced this promising germplasm in readiness for the next generation. [ABSTRACT FROM AUTHOR]

Apple industries suffer from major apple diseases because of widely planted susceptible cultivars. Developed disease-resistant cultivars that often carry only a single source of resistance are not expected to be durable over time. Cultivars with multiple sources of resistance are often commercially unacceptable due to unsatisfactory fruit quality alleles inherited from unimproved and improved parents. To improve fruit quality, approximately five modified backcrossing generations have been used with phenotypic selection for offspring with the least proportion of unimproved genome and elite fruit quality. Modified backcrossing is time-consuming owing to the long juvenility of apple. Unimproved parents are always assumed to carry undesirable alleles in addition to the targeted resistance allele. To efficiently identify favorable offspring each generation, DNA-based markers would be useful. Locus-specific DNA tests are unavailable to detect many sources of resistance alleles. Known numbers of DNA segments from unimproved parents could help subsequent parent selection among offspring because of the direct connection to the probability of eliminating such segments each generation. Accurately estimating the proportion and number of unimproved segments requires precise information on genomic positions of recombinations that can be detected with effective genetic marker sets. High-resolution and genome-wide apple SNP arrays can be used to characterize unimproved DNA segments present in disease-resistant offspring to efficiently achieve durable resistance and elite fruit quality. To hasten apple flowering, a rapid generation cycling approach with transgenic genetic stocks is available. Using these tools is expected to effectively exploit additional unimproved germplasm toward apple genetic improvement. [ABSTRACT FROM AUTHOR]

Apple mildew, caused by the fungus Podosphaera leucotricha, is an ongoing problem for apple growers in most world production regions. To manage apple foliar mildew with less fungicide, use of mildew-resistant cultivars is an ideal option. The mildew resistance locus of crabapple 'White Angel' was previously named Pl-w and genetically mapped, with the closest DNA markers at least 4.6 cM away on one side of the locus. The objective of this study was to develop a DNA test with markers flanking Pl-w that can predict mildew susceptibility in 'White Angel' descendants. Two families with 'White Angel' as a common parent and 'Golden Delicious' and 'Fuji' as the other parents were screened for mildew susceptibility in a growth chamber. These seedlings were genotyped with reported DNA markers in the genetic vicinity of Pl-w. Segregation ratios in the two families fitted 1:3 and 3:5 ratios for resistance:susceptibility of seedlings. The DNA test developed, Md-Plw8-SSR, contained two SSR markers, CH01e12 and CH02g09, estimated at 0.01 ± 1.0 cM and 5.0 ± 1.0 cM, respectively, flanking the Pl-w locus. Md-Plw8-SSR identified individuals resistant to foliar mildew with 99.9% accuracy. The segregation ratios suggested mildew resistance from 'White Angel' was controlled by two unlinked loci with an unknown frequency and distribution in elite germplasm of the resistance allele at the second locus. When used in seedling screening, the new DNA test can eliminate at least some susceptible individuals but cannot detect allelic variation at the second, unmapped locus to ascertain resistance. Combining Md-Plw8-SSR with DNA tests for other sources of mildew resistance is expected to improve the ability to breed for durable resistance in apple. [ABSTRACT FROM AUTHOR]

Apple trees have a long juvenile period, which makes apple genetic improvement via breeding costly and time-consuming. Transgenic genetic stocks carrying the early-flowering gene BpMADS4 have been used to reduce the juvenility of apple from five or more years to less than 10 months. One such genetic stock, T1190, has been used widely in breeding and research. It was reported to be a seedling of 'Pinova' but the other parent was unknown. Not knowing the alleles that this unknown parent contributed to T1190 brings uncertainties to breeding programs and research studies. In this study, the full pedigree of the genetic stock T1190 was reconstructed using an apple 20K SNP array and a panel of 530 reference cultivars and breeding selections. T1190 was determined to be an offspring of 'Pinova' and 'Idared'. The full pedigree of T1190 was used to deduce the mosaic ancestor composition of the transgene-hosting chromosome. Such knowledge is useful to ensure breeding programs and research studies achieve their expected objectives. [ABSTRACT FROM AUTHOR]