Evaluation of Fire Blight Resistance in Apple Rootstocks Using Artificial Inoculation of Erwinia amylovora in In Vitro Cultured Plantlets and Acclimatized Seedlings

Fire blight, caused by Erwinia amylovora, is a destructive bacterial disease affecting over 200 species in the Rosaceae family, including economically significant crops such as apple, pear, apricot, and plum. It infects leaves, flowers, branches, and fruits, often leading to the complete loss of trees. Since its first outbreak in Korea in 2015, fire blight has spread to 1,044 hectares across 28 cities and counties as of 2022. With no curative treatments, current management in Korea relies on preventative measures, such as antibiotic applications before and after bloom and the removal of cankered tissues in winter. In contrast, the United States uses resistant rootstocks, such as the Geneva series, to control outbreaks. However, widely used Korean rootstocks, including M.9 and M.26, are highly susceptible, highlighting the urgent need for resistant alternatives. This study evaluated fire blight resistance in apple rootstocks under controlled laboratory conditions. In vitro assays identified 106 colony forming unit/ml as the optimal inoculum concentration, allowing significant resistance differentiation by day 28. Polymerase chain reaction analysis of acclimated seedlings revealed that resistant rootstocks G.11 and G.30 effectively restricted bacterial spread beyond the inoculation site, while the susceptible M.9 exhibited infection across most tissues. These findings highlight the potential of resistant rootstocks to mitigate fire blight and support sustainable apple production.