The 2012 ASHS Annual Conference

Fire blight, caused by the bacterium Erwinia amylovora, is a devastating disease of apple (Malus) that can kill young trees outright or result in permanent structural damage.  Several sources of resistance to fire blight have been described in apple (Malus), however most are from wild species or cultivars with inferior fruit quality. The goal of this project is to identify and characterize additional sources of resistance from both M. sieversii and M. x domestica with fruit quality traits suitable for use in scion breeding programs. What distinguishes M. sieversii from other wild Malus species as an excellent source of disease resistance for scion breeding is its large and palatable fruit. All data available within the USDA–ARS, Germplasm Resources Information Network (GRIN) database was used to select 200 M. sieversii accessions with a predicted 1:1 ratio of fire blight susceptible to resistant accessions and superior fruit quality traits for flavor, firmness, juiciness, and soluble solids, harvest date (favoring later dates) and resistance to apple scab. Replicated plantings of the accessions and control cultivars have been established at the WSU-TFREC, Wenatchee, WA and USDA–ARS–AFRS, Kearneysville, WV, and will be phenotyped for resistance to fire blight following inoculation with E. amylovora.  The accessions will also be genotyped using Genotyping By Sequencing for marker development. ‘Splendour’ is a commercial apple cultivar from New Zealand with excellent flavor that has been observed to be fire blight resistant under conditions of natural infection. To evaluate ‘Splendour’ as a potential donor of fire blight resistance, replicate plants of a subset of a ‘Honeycrisp’ x ‘Splendour’ population were bench grafted and evaluated for resistance to fire blight following inoculation with E. amylovora in the greenhouse.  Six of 21 plants appeared resistant suggesting resistance to fire blight was segregating within the population. Two additional populations of ‘Gala’ x ‘Splendour’ (500 individuals) and ‘Splendour’ x ‘Cripps Pink’ (Pink Lady) (250 individuals) were created and will be used to determine if a QTL for resistance to fire blight can be identified within ‘Splendour’. Plans are also underway to phenotype the RosBREED Malus Crop Reference Set and Washington State University Breeding Pedigree Sets to facilitate pedigree mapping of resistance to fire blight and marker development. After validation, molecular markers developed in the project will be made available to tree fruit breeding programs to facilitate the development of fire blight resistant apple and pear cultivars by marker assisted breeding.