Submitted to: Tree Genetics and Genomes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/6/2026
Publication Date: 8/12/2026
Citation: Clare, S., Zurn, J.D., Driskill, M., Montanari, S., Kirk, C., Lopez-Girona, E., Bus, V., Chagne, D., Gottschalk, C.C., Bassil, N.V. 2026. Validation and marker development for a chromosome 2 QTL for fire blight resistance in pear. Tree Genetics and Genomes. 22(25). https://doi.org/10.1007/s11295-026-01754-9.
DOI: https://doi.org/10.1007/s11295-026-01754-9

Interpretive Summary: Fire blight, caused by a bacterial pathogen, is a persistent problem for pear growers across most production regions around the world. Growing resistant varieties is one of the best options for managing fire blight. The resistant European cultivars ‘Potomac’ and ‘Old Home’, and the hybrid selection NJA2R59T69 were used in a previous study to identify chromosomal regions linked to the resistance. The major chromosome 2 region identified in the ‘Potomac’ and ‘Old Home’ sources overlap with previously reported regions that were previously identified in ‘Harrow Sweet’ and ‘Moonglow’, while that of NJA2R59T69 mapped to a nearby location on chromosome 2. In the current study, genes associated with disease resistance in the two chromosome 2 regions were sequenced in 23 accessions representing resistant and susceptible cultivars, and progeny from the three sources. Variants unique to resistant cultivars in these genic regions were targeted and used to genotype a diversity set of 467 pear accessions with known fire blight disease responses from multiple sources. Association mapping was conducted across subsets of the 467 accessions based on the shared fire blight resistance source. In the 48 accessions with the ‘Potomac’ source of resistance, association mapping identified a DNA variant that was predictive with 88.0% and 96.0% accuracy in ‘Potomac’ and ‘Pai Li’, respectively. However, it was only 68.9% and 67.2% accurate in ‘Old Home’ and the remaining resistant accessions, respectively. This marker was converted to a breeder-friendly assay that was predictive in both ‘Pai Li’ and ‘Potomac’ -derived accessions but not ‘Old Home’-sourced resistance. Resistance appears to be dominant, with the majority of accessions being heterozygous in this important region. A large proportion of resistant Asian accessions are homozygous for this resistance allele, suggesting these may be excellent sources to introgress resistance. Future work should look to further refine the chromosome 2 resistance region and determine which variant corresponds to each unique source of fire blight resistance.

Technical Abstract: Fire blight, caused by the bacterial pathogen Erwinia amylovora, is a persistent problem for pear (Pyrus spp.) growers across most production regions around the world. Growing resistant varieties is one of the best options for managing fire blight. The resistant P. communis cultivars ‘Potomac’ and ‘Old Home’, and the hybrid selection NJA2R59T69 were used in a previous study to identify quantitative trait loci (QTLs) linked to the resistance. The major chromosome 2 QTLs identified in the ‘Potomac’ and ‘Old Home’ sources overlap with QTLs that were previously identified in ‘Harrow Sweet’ and ‘Moonglow’, while that of NJA2R59T69 (through P. ussuriensis ‘Pai Li’) mapped to a nearby location on chromosome 2. In the current study, genes associated with disease resistance in the two chromosome 2 QTL regions were cloned in 23 accessions representing resistant and susceptible cultivars, and progeny from the three sources. Alleles unique to resistant cultivars in these genic regions were targeted by SeqSNP and used to genotype a diversity set of 467 pear accessions with known fire blight disease responses from multiple sources. Association mapping was conducted across subsets of the 467accessions based on the shared fire blight resistance source. In the 48 accessions with the ‘Potomac’ source of resistance, association mapping identified a marker (Chr2_3601869) that was predictive with 88.0% and 96.0% accuracy in ‘Potomac’ and ‘Pai Li’, respectively. However, Chr2_3601869 was only 68.9% and 67.2% accurate in ‘Old Home’ and the remaining resistant accessions, respectively. This marker was converted to a PACE assay that was predictive in both ‘Pai Li’ and ‘Potomac’ -derived accessions but not ‘Old Home’-sourced resistance. Resistance appears to be dominant, with the majority of accessions being heterozygous at the Chr2_3601869 locus. A large proportion of resistant Asian accessions are homozygous for the resistance allele at Chr2_3601869, suggesting these may be excellent sources to introgress resistance. Future work should look to elucidate the critical recombination points within the three mapping populations to further refine the chromosome 2 resistance loci and determine which allele/haplotype corresponds to each unique source of fire blight resistance.