2019 ASHS Annual Conference

Fire blight, caused by Erwinia amylovora, is the most devastating bacterial disease of pear (Pyrus spp). This disease is a constant problem for the primary pear production areas of the U.S. Planting resistant cultivars is the best option for managing fire blight. Unfortunately, all major cultivars grown in the U.S. are susceptible to this disease. The cultivars Potomac and Old Home and the selection NJA2R59T69 are resistant to fire blight and can be used to create new cultivars with traits desired by growers and consumers. Therefore, three mapping populations (‘El Dorado × ‘Potomac’, ‘Old Home’ × ‘Bartlett’, and NJA2R59T69 × ‘Bartlett’) were developed to identify genomic regions associated with fire blight resistance. The populations were initially genotyped with a DNA fingerprinting set to identify progeny that were the result of an unexpected pollination event. Progeny which had the expected pedigree were selected for continued analysis and were genotyped with the recently developed Axiom Pear 70K Genotyping Array. In 2017 and 2018, multiple actively growing shoots of field grown seedling trees were inoculated with E. amylovora E153n via the cut leaf method. The percentage of current season’s shoot that was blighted was calculated. The phenotypic distributions for each population were severely skewed toward resistance and the number of genes mediating resistance could not be easily discerned. Chromosomal linkage groups were established for each population using a cross-pollinating mapping approach with JoinMap 5. Each map had approximately 30,000 markers distributed across the whole genome. The average total length of the high quality maps was 1,087 cM. An integrated two-way pseudo-testcross approach was used to map QTLs using MapQTL 6. A single significant QTL (α = 0.05) mediating resistance was identified for each population in a similar region on chromosome 2. Fire blight resistance QTLs have been previously reported in this region for ‘Harrow Sweet’ and ‘Moonglow’ (a parent of ‘Potomac’). The presence of the chromosome 2 QTL in NJA2R59T69 is interesting as the resistance originated from the P. ussuriensis selection ‘Illinois 76’ and not a P. communis source like ‘Potomac’ and ‘Old Home’. This could suggest that the resistance in this region may be due to a conserved basal defense response. Additional work is needed to further characterize this unique genomic region.