Copper Resistance in Pseudomonas syringae pv. actinidiae Strains from the Prefecture of Pella, Northern Greece, and a Comparison of the Effectiveness of Several Commercial Products to Control

This study highlights the growing concern over copper resistance in Pseudomonas syringae pv. actinidiae ( Psa ), a significant pathogen threatening kiwifruit cultivation globally. Copper-based treatments, widely used for their broad-spectrum bactericidal properties, have become less effective over time due to the emergence of copper-resistant strains. This study found that 22 strains of Psa , collected from Greek kiwifruit orchards, exhibited significant copper resistance. All strains were capable of growing in copper concentrations of at least 200 µg/mL (0.8 mM), and two strains (9.1%) survived even at higher concentrations of 400 µg/mL (1.6 mM). Additionally, resistant bacterial colonies were observed in four strains (18.2%), suggesting the presence of resistant subpopulations within the broader bacterial community. In addition to studying copper resistance, the research evaluated the effectiveness of several products in controlling Psa through field experiments. The copper products IONIC CONCENTRATED COPPER, MAGNA BLUE, and COPROFIX ULTRA were tested due to their historical effectiveness in managing bacterial diseases in plants. However, these copper products were unable to fully control Psa in this study, likely due to the observed resistance. In addition, the plant defense activator products MICONIC and NUTRI BIOCLEAN, NUTRI BIOCLEAN, BAR, and BION were evaluated and showed promising results in reducing the intensity of symptoms associated with Psa , suggesting that these formulations may enhance the plant’s natural defenses against the pathogen. While these products did not completely eliminate the disease, they significantly reduced symptom severity. The findings highlight the need for alternative strategies to combat Psa , particularly in areas where copper resistance has developed.