Survival of Pseudomonas syringae pv. actinidiae in the Environment

Pseudomonas syringae is a group of bacteria that can survive in the environment outside their host plants, the environment constituting a reservoir for the pathogen. To determine whether Pseudomonas syringae pv. actinidiae (Psa), the causal agent of bacterial canker of kiwifruit, could survive in the environment, and where in the environment it could survive, experiments were conducted in the physical containment level 2 laboratory at Plant & Food Research at Ruakura, and some samples from the environment were also analysed for presence of Psa. While Psa survived very well as an epiphyte on its host plant (kiwifruit leaves and flowers), it was not able to multiply on other plants that were examined (common trees of New Zealand shelterbelts, and some weeds present in kiwifruit orchards). Since Psa was isolated from kiwifruit pollen, the potential role of pollinating insects such as honey bees as a vector was also examined. Psa was found to survive for several days in a beehive in which Psa was artificially introduced, and honey bees which were foraging in infected orchards were found carrying Psa. In water, Psa survived and multiplied consistently for long periods of time only in sterile rainwater. In sterile soils, Psa could survive but could not colonise. On compost, Psa populations rapidly decreased below the threshold of detection, whether the composts had been sterilised or not. Some natural antagonists to Psa have been identified from the compost analysed; however, even in absence of such antagonists Psa could not survive for more than a few days. This increased understanding of the epidemiology of Psa should help with devising control methods, which will reduce the inoculum in the environment and ultimately will decrease disease incidence in kiwifruit orchards.