Bacteriophages Biocontrol of Kiwifruit Bacterial Canker Caused by Pseudomonas syringae pv. actinidiae (Psa) in Two Seasons Under Field Conditions

<b>Background:</b> Since 2008, the kiwifruit industry has been significantly impacted by <i>Pseudomonas syringae</i> pv. <i>actinidiae</i> (Psa), the agent responsible for bacterial canker in kiwifruit. Existing treatments, such as copper-based compounds and antibiotics, have faced challenges related to resistance and soil contamination. Phage therapy is a promising and safe alternative for controlling this pathogen. This study aimed to evaluate the use of a mixture of four isolated and characterized bacteriophages as potential biocontrol agents against Psa. <b>Methods:</b> Trials were conducted at two locations in Chile, where Psa presence was reported during the 2019/2020 and 2020/2021 seasons, with a focus on the spring stages. Different formulations were tested each season to evaluate possible improvements in effectiveness. <i>Pseudomonas</i> spp. isolates obtained from epiphyte populations were characterized using morphological, biochemical (LOPAT), and molecular techniques. <b>Results:</b> Field trials demonstrated that the phage mixture effectively reduced the damage associated with Psa on kiwi leaves, resulting in a decrease in the <i>Pseudomonas</i> spp. bacterial load (42.9% for Peumo and 25% for Linares) at both locations during the first season trials. This decrease is associated with a reduction in the incidence and severity of the disease in kiwi plants in the Peumo orchard. In both seasons, bacteriophages reduce Psa symptoms in treated kiwi plants compared to untreated controls, at least at one location and evaluation. In both orchards during the first season, bacteriophages also outperformed copper- and antibiotic-based treatments used by farmers. Bacteriophage therapy is eco-friendly and safe for both applicators and consumers.