Influence of copper on the growth of Pseudomonas syringae pv. actinidiae strains isolated from kiwifruit orchards affected by the bacterial canker

The control of Pseudomonas syringae pv. actinidiae (Psa), the causal agent of the bacterial canker of kiwifruit, is currently based on the use of good agronomic practices and of copper compounds. In several phytopathogenic bacteria (P. syringae, P. syringae pvs. tomato and syringae, X. campestris pvs. vesicatoria and juglandisls) it has been established the induction of copper resistance, which could limit or impair the effectiveness of copper-based treatments. The molecular and genetic basis of the copper resistance has been studied in detail in P. syringae pv. tomato and the copper-resistance genes, located on plasmids (pPaCu), have a high degree of homology in different Pseudomonas species. In the literature it was shown that in copper-resistant strains of Psa has been found one or two pPaCu plasmids which carried genes for copper and streptomycin-resistance. In kiwifruit orchards frequent copper treatments may induce the selection of Psa populations resistant to copper with serious consequences for kiwifruit producers. In this study, the sensitivity to copper of Psa strains isolated in the Emilia-Romagna region in 2009 in kiwifruit orchards affected by the bacterial canker was investigated. The growth of 9 Psa strains in liquid culture media containing copper nitrate [Cu(NO3)2] at concentrations ranging from 0 to 4.25 mM was followed over time and for each strain the MIC (minimum inhibitory concentration) was determined. 24h after the treatment, the growth was inhibited by 3.5 mM Cu(NO3)2 in 6 strains and by 3.75 mM in one strain, however in 5 of those strains the antimicrobial effect was bacteriostatic.