Characterizing the host response and genetic control in 'Honeycrisp' to apple scab (Venturia inaequalis)

Abstract

Two novel apple scab resistance loci have been identified in the apple cultivar Honeycrisp, an emerging cultivar in North America that is utilized in apple breeding programs worldwide. Greenhouse inoculation experiments with the apple scab fungal pathogen, Venturia inaequalis, identified a resistance defense response in `Honeycrisp' and its ancestors `Keepsake', `Frostbite', and `Northern Spy' seven days after inoculation. The defense response ranged from necrotic and chlorotic lesions to stellate necrosis and class 3b lesions (with sporulation). A hallmark of the resistance defense response is autofluorescence at the infection site in cleared leaf tissue. Several `Honeycrisp' progeny populations were screened with monoconidial isolates of V. inaequalis and segregated 3:1 for resistance, suggesting two resistance genes inherited from `Honeycrisp'. A consensus `Honeycrisp' linkage map with 1091 SNP markers was constructed for use in mapping. Two resistance loci were mapped using linkage mapping and quantitative trait loci mapping approaches. Marker haplotypes were constructed to trace the inheritance of resistance loci. Rvi19 mapped on linkage group 1 at ~50 cM. In the Rvi19 haplotype, the 138 bp allele for the Ch-Vf1 marker cosegregates with resistance, and is identical by state (IBS) with the Rvi17 resistance in `Antonovka'. Rvi19 is transmitted from `Frostbite' to `Keepsake' to `Honeycrisp', and into the resistant progeny of `Honeycrisp'. The other locus, Rvi20, mapped onto linkage group 15, and is IBS with a marker haplotype found in the susceptible cultivar Golden Delicious. Rvi20 is transmitted to `Honeycrisp' from an unknown parent. Molecular marker haplotypes were used to identify advanced selections in the University of Minnesota breeding program with pyramided scab resistance. Candidate genes were identified at each haplotype that can serve as starting points for identifying the functional genes conferring resistance. A collection of V. inaequalis isolates was assembled and curated from six locations in Minnesota for screening scab resistance. These 80+ isolates were examined for genetic diversity and population structure and provide a snapshot of the diversity of the pathogen present at this time.