Analysis of kiwifruit bleeding sap: a useful method for early detection of Pseudomonas syringae pv. actinidiae

The bacterial canker of kiwifruit, caused by Pseudomonas syringae pv. actinidiae (Psa), became considerably important during last five years, after severe epidemics occurring from 2008/09 in central and northern Italy. The rapid spreading of the disease during last few years highlighted the need of a fast and sensitive method for the Psa detection in symptomless trees, to find possible inoculum sources and therefore limit the pathogen spreading. In this study we analyzed 51 samples of bleeding sap from asymptomatic trees of Actinidia deliciosa and A. chinensis collected in early spring 2012. The bleeding sap samples were used both for direct PCR assays and for bacteria isolations on NSA medium. The recovered Psa-like isolates were purified and characterized by the following biochemical tests: levan production, presence of oxidase, soft rot activity on potato disks, presence of arginine dehydrolase, hypersensitivity reaction in tobacco leaves (LOPAT) and fluorescence on King’s medium B; isolates were also assayed by PCR. Direct PCR assays on crude bleeding sap samples were able to detect the presence of the pathogen in three of the 51 tested samples, while the direct isolations detected the pathogen in six samples. Although more samples should be analyzed to evaluate the robustness of the method, analysis of kiwifruit bleeding sap may be a useful, rapid and sensitive tool for screening Psa-infected plants and/or to monitor the spreading of the pathogen in the field