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.

Olive knot disease, caused by Pseudomonas savastanoi pv. savastanoi, is a significant economic threat to olive cultivation in Mediterranean regions. This study focused on the polyphasic characterization of fifty-five P. savastanoi (Psav) isolates from various hosts (olive, oleander, myrtle, and jasmine) isolated in Crete, employing phenotypic, molecular, and copper resistance analyses. Biochemical profiling using LOPAT and API20 NE tests confirmed 100% similarity between olive, jasmine and oleander isolates and the type strain CFBP1670^T. Molecular characterization (BOX- and iaaL-PCR) revealed limited genetic variation among most isolates, with minor variations in myrtle, oleander, and Spanish broom strains. Multilocus sequence analysis further supported the close genetic relatedness of the Cretan isolates, while revealing limited internal differentiation. Copper resistance assays showed that most isolates were moderately sensitive (0.4 mM), while one oleander and one jasmine isolate were resistant (≥0.8 mM). In vitro evaluation of copper formulations showed that copper hydroxide, Bordeaux mixture, and tribasic copper sulfate effectively inhibited Psav growth, while copper oxide and copper oxychloride were largely ineffective. Finally, the copper resistance genes copA, copB, copD, copR, copS, and cusA were identified in all tested isolates regardless of their individual copper sensitivity levels, whereas copC, cusB, and cusC were not detected. This suggests that phenotypic variations in copper sensitivity likely stem from differences in gene expression rather than the mere presence or absence of these specific genes. These findings enhance the understanding of Psav diversity and provide a robust framework for optimizing sustainable management strategies for olive knot disease in Crete.