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Phage therapy to manage bacterial canker in kiwifruit caused by Pseudomonas syringae pv. actinidiae

Chang-Sik Oh: Department of Horticultural Biotechnology, Kyung Hee University

<div>Bacterial canker in kiwifruit trees caused by <em>Pseudomonas syringae </em>pv.<em> actinidiae</em> (<em>Psa</em>) is very problematic worldwide. Due to the limitation of usage of agricultural antibiotics or copper compounds for disease management, alternative methods need to be developed. Recently, bacteriophages, viruses that specifically kill target bacteria, have been reconsidered as potential biological control agents for bacterial pathogens due to their specificity and safety to environment. In this study, 77 bacteriophages effective to <em>Psa </em>strains, including the Psa2 and Psa3 groups, were isolated from soils collected from kiwifruit orchards in South Korea, and seven bacteriophages were selected based on restriction enzyme digestion patterns of genomic DNA and their host range. Two belonged to the <em>Myoviridae</em> family and five belonged to the <em>Podoviridae</em> family, based on their morphology observed with transmission electron microscope. They were effective to not only <em>Psa </em>strains, but also other diverse <em>P. syringae</em> pathovars. Lytic activity of the selected bacteriophages was sustained <em>in vitro</em> until 80 hours, and their activity remained stable up to 50<sup>o</sup>C, pH 11, and under UV-B light. To examine their control efficacy against <em>Psa</em>, bacteriophage cocktails mixed with three or five bacteriophages were sprayed to one-month-grown grafted kiwifruit trees in the greenhouse 3 hours before spraying with <em>Psa</em>. Treatment of bacteriophage cocktails with 10<sup>8</sup> plaque forming units/ml could protect kiwifruit trees from <em>Psa</em> up to almost 90% of Agrimycin® control efficacy. These results indicate that selected bacteriophages could be useful for phage therapy against <em>Psa</em> in kiwifruit trees and also probably other <em>P. syringae</em> pathovars.</div>

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