About Psa

Pseudomonas syringae pv. actinidiae (Psa) is a bacteria that can result in the death of kiwifruit vines. Psa carries no risks associated with human or animal health and does not affect plants other than kiwifruit vines. Psa is believed to be spread by weather events, namely wind and rain, and plant material. It is also believed to be spread by footwear, vehicles and orchard tools. In an orchard it can exist as:

• an epiphyte, living on plant surfaces without causing—high levels of—infection; and/or
• as an endophyte, living within the vine, having entered through natural  plant openings or man made wounds—resulting in severe infection.

Growth of the bacteria outside/inside the vines can result in leaf spotting, cane/leader dieback and, in extreme cases, vine death accompanied by the production of exudates.

Causes of Psa (from the KVH/ZESPRI R&D programme) 30.03.12

Pseudomonas syringae pv. actinidiae (Psa) is a pathogenic bacterium of Actinidia (kiwifruit) species. A number of genes have been identified in Psa—in particular the virulent form Psa-V now established in New Zealand—that code for proteins that may allow the bacteria to infect the kiwifruit plant host, colonize tissues and live within the vine. From a biological perspective, Psa-V  derives both nutrition and protection from environmental stresses from the vine and is the causal agent of a number of disease symptoms including leaf spots and necrosis, flower wilting, cane die-back, branch and trunk cankers and even plant death.

With regards to the infection process, recent genomic studies have shown that strains isolated in Italy and similar to Psa-V possess genes that code specifically for proteins involved in the inhibition of nitric oxide metabolism within the host plant. Nitric oxide is known to play a fundamental role in plant disease resistance and thus by impacting its production Psa is able to decrease one of the plant immune responses that may otherwise aid to inhibit Psa host colonisation. Genes have also been identified in Psa strains that promote woody tissue degradation indicating the potential of Psa to breakdown the actual woody component of a host kiwifruit vine.

Work is currently underway to determine the interaction between Psa-V and host kiwifruit plants that brings about tissue wilting and cane death. Anecdotal evidence suggests that the xylem, as a result of the colonisation of the conductive tissue and the production of high levels of bacteria, may become blocked inhibiting water movement within the vine resulting in tissue wilting and eventually tissue death. The potential role of toxins in this process has yet to be established so too is the potential of Psa-V to produce toxins that may hasten host tissue die back or cause vine collapse.

To date there is no evidence to suggest that Psa-V interacts directly to alter the DNA of the host plant or induce mutations that result in disease symptoms. For a review of recent advances in the understanding of Psa/ kiwifruit interactions refer to the R&D section of the KVH website.