Investigating a New Psa Variant in New Zealand
Abstract
A new genetic variant of the kiwifruit vine pathogen Pseudomonas syringae pv. actinidiae (Psa), the cause of bacterial canker in kiwifruit (Actinidia spp.), was identified in New Zealand during 2022. The variant, which is missing the hopF2 effector gene, was detected as part of routine testing of Psa isolates collected from two commercial orchards in New Zealand. A specific PCR primer was developed and the Psa variant was tested in a competition assay on tissue cultured A. chinensis var. chinensis ‘Hort16A’ and A. chinensis var. deliciosa (‘Hayward’) plantlets and found to out-compete wild-type Psa strains on ‘Hort16A’ but not on ‘Hayward’. The competition assay was repeated using A. chinensis var. chinensis ‘Zesy002’ (commonly known as Gold3) and ‘Zes008’ (commonly known as Red19) tissue culture plantlets. The hopF2 Psa variant did not out-compete wild type Psa on ‘Zesy002’ but did on ‘Zes008’, although to a much lesser extent than on ‘Hort16A’. The two orchards of interest, where the original hopF2 Psa variants were first detected, and nearby properties, had additional leaf samples collected to monitor the persistence and spread of the variant. Trap plants were also utilised to increase the recovery of Psa from the two orchards of interest by suspending potted ‘Hayward’ plants beneath the vine canopy during predicted rainfall events in spring 2022. Leaves that developed typical Psa spotting were collected, and Psa was isolated and tested using the new primers. Freshly collected strains of Psa (259) were screened and the variant was detected in just 6.6% of these. Research findings to date suggest the newly emerged hopF2 Psa variant is not significantly more pathogenic on ‘Zesy002’ and ‘Zes008’ following inoculation studies on leaves and stems of potted plants in containment glasshouses. On going monitoring and vigilance is recommended.
