Field Resistance to Pseudomonas Syringae pv. Actinidiae (Psa) in a Diploid Male Kiwifruit Breeding Parent Is Conferred By Multiple QTLs

The bacterium Pseudomonas syringae pv. actinidiae (Psa) has cost the NZ kiwifruit industry approximately 500 million dollars over five years.   The breeding programme needs to achieve quickly high levels of tolerance/resistance to Psa and develop marker assisted selection for the development of new cultivars. We identified QTLs for field resistance to Psa in a diploid Actinidia chinensis family resulting from a cross between a susceptible female (Hort16A) and a resistant male parent (P1) widely represented in commercial populations.   The small mapping population of 52 clonally-propagated individuals was phenotyped at monthly intervals (for 18 months) for disease indicators, including cane death and tip death.  Genotyping-By-Sequencing (GBS) was used to construct a genetic map of 27 of the 29 chromosomes of P1, spanning 4,295 cM and comprising 3,364 SNP markers. Kruskal Wallis analysis identified a number of QTLs contributing to resistance control, by both phenotypic measures.  Some of the QTLs for both traits were in common. The QTLs effects were additive, as resistance strength increased with the number of QTLs present in an individual.  SSR markers mined from the reference A. chinensis genome sequence enabled allelotyping of the QTL effect and will be used for QTL validation in existing phenotyped breeding populations.