Identification and Characterization of Pseudomonas syringae pv. syringae, a Causative Bacterium of Apple Canker in Korea

Abstract

In the present investigation, bacterial isolates from infected apple trees causing apple canker during winter were studied in the northern Gyeongbuk Province, Korea. The pathogen was identified as <i>Pseudomonas syringae</i> pv. <i>syringae</i> (<i>Pss</i>) through various physiological and biochemical characterization assays such as BIOLOG, gas chromatography of fatty acid methyl esters, and 16S rRNA. Bioassays for the production of phytotoxins were positive for syringopeptin and syringomycin against <i>Bacillus megaterium</i> and <i>Geotrichum candidum</i>, respectively. The polymerase chain reaction (PCR) method enabled the detection of toxin-producing genes, <i>syr</i>B1, and <i>syp</i>B in <i>Pss</i>. The differentiation of strains was performed using LOPAT and GATTa tests. <i>Pss</i> further exhibited ice nucleation activity (INA) at a temperature of –0.7°C, indicating an INA⁺ bacterium. The ice-nucleating temperature was –4.7°C for a non-treated control (sterilized distilled water), whereas it was –9.6°C for an INA^– bacterium <i>Escherichia coli</i> TOP10. These methods detected pathogenic strains from apple orchards. <i>Pss</i> might exist in an apple tree during ice injury, and it secretes a toxin that makes leaves yellow and cause canker symptoms. Until now, Korea has not developed antibiotics targeting <i>Pss</i>. Therefore, it is necessary to develop effective disease control to combat <i>Pss</i> in apple orchards. Pathogenicity test on apple leaves and stems showed canker symptoms. The pathogenic bacterium was re-isolated from symptomatic plant tissue and confirmed as original isolates by 16S rRNA. Repetitive element sequence-based PCR and enterobacterial repetitive intergenic consensus PCR primers revealed different genetic profiles within <i>P. syringae</i> pathovars. High antibiotic susceptibility results showed the misreading of mRNA caused by streptomycin and oxytetracycline.