Protection Efficacy of Antibacterial Strains against Fire Blight Caused by Erwinia amylovora on Apple Blossom

Fire blight caused by Erwinia amylovora is one of the destructive diseases in the family of Rosaceae plants, including apple and pear, in the world. Since the first report in 2015, the number of infected farms and area steadily increased in Korea. In case of eradication failure against this disease, protection strategies using both chemicals and biocontrol agents should be established. In this study, to select an effective antibacterial agent against fire blight on apple trees, four bacterial strains isolated from Jeju Island were investigated. Among the bacterial strains, Bacillus circulans BRH433-2 showed bactericidal effects against E. amylovora Ea385 forming inhibition zone on an artificial medium. The other bacterial strains such as Pseudomonas fluorescens THJ609-3, Micrococcus luteus TRK2-2 and P. fluorescens TRH415-2 showed bacteriostatic activity preventing growth of E. amylovora Ea385 in shaken cultures as well as on detached apple blossoms inoculated with E. amylovora Ea385, as measured with quantitative PCR. Bio-tests on detached blossoms showed that the treatment with all bacterial strains caused strong suppression of bacterial ooze formation, indicating inhibition of disease incidence of fire blight, which was similar to blossoms treated with streptomycin sulfate. Therefore, it was suggested that these bacterial strains may be useful in organic apple orchards to control fire blight where chemical use is limited.