DEVELOPING AND USING BIOASSAYS TO SCREEN FOR PSA RESISTANCE IN NEW ZEALAND KIWIFRUIT
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 prolifera¬tion 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. argute) 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.
Hoyte, S., Reglinski, T., Elmer, P., Mauchline, N., Stannard, K., Casonato, S., Ah Chee, A., Parry, F., Taylor, J., Wurms, K., Yu, J., Cornish, D. and Parry, J. (2015). DEVELOPING AND USING BIOASSAYS TO SCREEN FOR PSA RESISTANCE IN NEW ZEALAND KIWIFRUIT. Acta Hortic. 1095, 171-180
Pseudomonas syringae pv. actinidiae, Actinidia spp., woody-stem bioassay, green stem-stab bioassay, screening, susceptibility