Fire Blight Susceptibility of 20 Diverse Pear (Pyrus spp.) Rootstock Breeding Parents

Fire blight is a bacterial disease caused by Erwinia amylovora, which can cause devastating losses to pear (Pyrusspp.) growers. Infections can lead to a reduction in fruit yield, the need to remove some or all scion tissues, and entire tree death. Rootstocks with lower fire blight susceptibility can confer some degree of tolerance to suscep-tible scions. Since most U.S. pear cultivars are susceptible to fire blight infection, breeding low-susceptibility rootstock cultivars can help decrease losses for the pear industry. The Washington State University (WSU) Pear Rootstock Breeding Program was established to develop Pyrus rootstocks, with target traits such as dwarfing, pre-cocity, cold-hardiness and reduced fire blight susceptibility. This study evaluated fire blight response of 20 diverse accessions, as grafted scion tissue. Two greenhouse experiments were conducted in 2021 on up to 20 individuals per accession, which were arranged in a randomized complete block design with four blocks and five replicates. One actively growing shoot per tree was inoculated with E. amylovora strain 153n. Fire blight response was measured after disease progression stopped and was quantified as percent shoot length blighted (%SLB). Average accession responses ranged from 0.1 to 100 %SLB and were highly correlated between experiments (Pearson's r = 0.83, P <= 0.001). Individuals in Experiment B had significantly higher severity of infection; however, the relative order of accession based on severity was consistent with that of Experiment A. In both experiments, nine accessions consistently exhibited low fire blight susceptibility (0.1 to 10.9 %SLB), while six accessions had high fire blight susceptibility (35.2 to 100 %SLB). Results from this study provide insights for 20 potential breeding parents in the WSU Pear Rootstock Breeding Program.