Developing and using bioassays to screen for Psa resistance in New Zealand kiwifruit

Abstract

The bacterial pathogen Pseudomonas syringae pv. Actinidiae (Psa), biovar 3 causes a destructive disease of kiwifruit. Psa was first identified in New Zealand on 5 November 2010 on Actinidia chinensis 'Hort16A' (Zespri Gold) kiwifruit vines in the Te Puke region and has now spread to 81% of kiwifruit orchards. Intensive spray programmes based upon Actigard® and copper-based products have been implemented by growers, but the high usage of copper products is unsustainable. Therefore, breeding new cultivars with greater resistance to Psa has become a priority. Plant and Food Research (PFR) has developed a range of bioassays to characterise kiwifruit germplasm (e.g., tissue culture plantlets, potted seedling and mature vines) for resistance to Psa. The 'woody-stem bioassay' measures Psa lesion development and bacterial proliferation on wound-inoculated cane segments (bud wood). The measurements are combined to provide a woody stem bioassay index (WSBI) which ranges from 0 (resistant) to 100 (susceptible). Since its deployment in July 2012, the woody-stem bioassay has been successfully used to determine the Psa resistance status of five commercial cultivars, 75 pre-commercial clonal selections and >2,000 parental and seedling kiwifruit from the PFR breeding programme. A 'green stem-stab' bioassay was also developed to compare susceptibility of actively growing shoots. Soft stems or shoots are inoculated using a toothpick dipped in Psa suspension and then scored for a range of symptoms (e.g. lesion size and stem collapse) after 2-3 weeks incubation in high humidity. This bioassay has been used to evaluate the Psa-resistance status of seedlings from selected crosses, including populations being interrogated for molecular markers of Psa resistance. Seedlings from inter specific-crosses (A. chinensis × A. arguta) exhibited greater resistance to Psa than A. chinensis × A. chinensis seedlings. These bioassays are currently being utilised for identifying genotypes within the PFR kiwifruit breeding programme that are resistant to Psa.