Detection of Pseudomonas syringae pv. actinidiae, causal agent of bacterial canker of kiwifruit, from symptomless fruits and twigs, and from pollen

Abstract

Pseudomonas syringae pv. actinidiae (Psa), the causal agent of bacterial canker of kiwifruit, was monitored in symptomless fruits, twigs and pollen of the host using bacterial isolation and DNA-extraction followed by two PCR-assays (direct-PCRs). A procedure for Psa detection from symptomless twigs was established. Out of 16 symptomless twigs samples, Psa was detected in 12 samples by isolation and 13 samples by direct-PCR. Thirteen pollen samples were treated using two different procedures; Psa was detected in eight samples by isolation and ten samples by direct-PCR. By washing 108 samples of fruits, Psa was detected by isolation in only two samples, collected from severely affected orchards. However, one of these samples contained wilted fruits, whereas for the other, only one colony was isolate. From 60 bulk-samples of fruits, endophytic Psa was detected in six samples by isolation and ten samples by direct-PCRs. A Psa-positive bulk-sample of fruits was analyzed separately as individual fruits: there was a faint signal in five or seven fruits out of 50 depending on the PCR assay used. Isolation was negative for these samples. Presence of the pathogen on bulk-fruit samples could be due to low amounts of inoculum distributed over many fruits: as a consequence, there is a negligible risk of introducing the pathogen into countries free of bacterial canker. This integrated approach (isolation plus PCR) is proposed as a tool for the analysis of symptomless kiwifruit material for the presence of Psa.