Saccharin induces resistance against Pseudomonas syringae pv. actinidiae (Psa biovar 3) in glasshouse kiwifruit and orchard vines

The artificial sweetener saccharin has been reported to enhance resistance against pathogen attack in various plant species. In this study, foliar application of saccharin resulted in increased resistance to leaf infection by Pseudomonas syringae pv. actinidiae biovar 3 (Psa) in two Actinidia chinensis cultivars, Hayward and Zesy002. In glasshouse plants, the application of saccharin at 0.25, 0.5, 1.0 and 2.0 g/L, 1 week before inoculation with Psa, induced a dose‐dependent reduction in leaf necrosis in both cultivars. Saccharin at 2.0 g/L reduced leaf necrosis in Hayward by 77% and in Zesy002 by over 90%. However, saccharin (2.0 g/L) did not inhibit growth of Psa in liquid media, thus suggesting induced resistance (IR) as the primary mode of action against leaf infection. The development of IR in both cultivars was concomitant with the accumulation of salicylic acid (SA) and salicylate glycoside (SAG), and the upregulation of SA‐pathway genes (PR1 and PR2) in treated leaves. In orchard‐grown Hayward vines, saccharin (1.0 g/L) induced the upregulation of SA‐pathway genes and reduced Psa leaf necrosis and flower bud rot by 50% and 25%, respectively, compared with controls. However, saccharin residues were detected in fruit collected from vines that were sprayed with saccharin (1.0 g/L) before flowering. Residue level correlated with application frequency and timing and was highest (0.051 mg/kg) in vines that received three preflowering sprays. This exceeds the default maximum residue limit (MRL; 0.01 mg/kg) for some export markets and could limit the use of saccharin for disease management.