Kiwifruit Xylem Sap: Composition and In Vitro Growth of a Virulent Strain of Pseudomonas syringae pv. actinidiae

Pseudomonas syringae pv. actinidiae (Psa) biovar 3 (also known as Psa-V) is a devastating bacterial pathogen of kiwifruit vines, and the causal agent of bacterial canker. The disease causes symptoms ranging from leaf spotting to woody cankers that produce exudate. Several Pseudomonas syringae pathovars are able to colonise distant plant tissues by systemic movement within the host. Published studies of kiwifruit Psa suggest bacteria may spread systemically through xylem vessels. The aim of this work is to understand how xylem sap composition, and therefore nutrient availability, can affect the ability of Psa biovar 3 to proliferate in xylem. We have examined the composition of the xylem saps of different kiwifruit cultivars throughout the season at three sites in New Zealand and using in vitro growth studies, tested the ability of a virulent Psa biovar 3 strain to proliferate on these xylem sap samples. Our results show that xylem sap metabolite profiles change both during the season and between cultivars. In vitro Psa growth in xylem sap was dependent on the plant phenological stage and cultivar. This information will be used to develop management practices to manipulate Psa-host interactions and to select more tolerant cultivars.