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Acibenzolar-S-methyl (ASM), the active ingredient in Actigard® (Syngenta), is a plant defence elicitor used for the management of bacterial canker caused by  Pseudomonas syringae  pv.  actinidiae  ( Psa ) in kiwifruit. We examined changes in gene expression and phytohormone content in two kiwifruit cultivars, ‘Hayward’ ( Actinidia chinensis  var.  deliciosa ) and ‘Zesy002’ ( A. chinensis  var.  chinensis ), following ASM application. In glasshouse studies, ASM-induced resistance to stem inoculation with  Psa  was accompanied by upregulation of salicylic acid (SA) defence pathway genes ( PR1, PR2 , and  PR5 ) in stems and leaves. The expression of  PR1  in ASM-treated plants increased by c. 5-fold in ‘Hayward’ and by over 15-fold in ‘Zesy002,’ relative to the untreated controls, whereas  PR2  and  PR5  increased by up to 4-fold in both cultivars. Orchard studies over three seasons confirmed the utility of  PR1, PR2 , and  PR5  for monitoring ASM-induced responses in mature vines.  PR1  and  PR5  were more strongly induced by ASM than  PR2  and gene upregulation in ‘Hayward,’ was accompanied by a reduction in Psa leaf spotting; no such relationship was measurable for ‘Zesy002’ because leaf spot symptoms are rarely expressed. In the third season, six additional gene candidates,  BAD, Gluc2, Class IV Chit, EDS1A, NPR3 , and  NIMIN2 , were responsive to ASM in ‘Hayward’ and ‘Zesy002.’ Gene upregulation was transient, with expression levels increasing by 1 d after ASM application and declining to control levels between 7 and 14 days. Moreover, the amplitude of gene upregulation depended on leaf developmental stage and was greater in the first true leaf and the youngest mature leaves than in immature leaves along the same shoot. Phytohormone content did not show a repeatable response pattern to ASM in potted plants or in vines possibly as a consequence of their wider role in regulating plant growth and mediating environmental responses. In conclusion, this study demonstrates that defence gene expression can be used to monitor responsiveness to ASM in two genetically distinct mature kiwifruit cultivars (‘Hayward’ and ‘Zesy002’) under orchard conditions. The use of defence marker genes could be of broader utility across kiwifruit species and could be used to guide ASM application schedules in the orchard.

Transient Changes in Defence Gene Expression and Phytohormone Content Induced by Acibenzolar-S-Methyl in Glasshouse and Orchard Grown Kiwifruit