Bacterial community associated with canker disease from sweet cherry orchards of central valley of Chile presents high resistance to copper

Abstract

The Chilean sweet cherry (Prunus avium (L.) L.) industry became the leading exporter worldwide. The bacterial canker is the most significant disease causing major economic losses. Pseudomonas syringae pv. syringae (Pss) is the only related pathovar recognized in Chile and it is mainly controlled with Cu-based antimicrobial compounds (CBAC). Soil contamination and the decreasing efficacy of CBACs by the emergence of Cu-resistant bacterial strains threaten the long- term sustainability of sweet cherry production. This study aimed at characterizing the bacterial community associated with canker infection injuries by assessing Cu resistance in sweet cherry orchards in the O’Higgins Region of Chile. Eighty bacteria isolates were obtained from tissue that presented bacterial canker symptoms from cherry trees. We assessed the production of fluorescent pigments in Fe-deficient media, presence of the housekeeping genes rpoD and cts, and presence of syringomycin-producing genes syrb and syrD. Their pathogenic ability was evaluated on immature sweet cherry fruits and Cu resistance was determined as the minimum inhibitory concentration (MIC) using CuSO4. Only five isolates were Cu-susceptible (MIC < 0.8 mM), while 75 isolates exhibited different levels of Cu resistance (MIC > 0.8 mM). At least one gene of the copABCD operon/or and its regulatory genes were detected in 17 isolates, suggesting that most isolates likely have different mechanisms of Cu tolerance. Six isolates were identified as Pss, presenting different degrees of Cu resistance, but all presented at least one feature of the copABCD operon. A bacterial community that presents high Cu-resistance, probably under control of diverse genetic mechanisms, decreases the efficacy of CBAC.