Evaluation of plant defence activators for the potential control of Pseudomonas syringae pv. aesculi

Abstract

Glasshouse trials were conducted using tobacco (Nicotiana benthamiana) to assess the efficacy of five plant defence activators (PDAs), namely harpin protein, potassium phosphite, salicylic acid analogue, silicon phosphite and a plant hormone–vitamin complex against the bacterial pathogen Pseudomonas syringae pv. aesculi (Pae), the causal agent of Pseudomonas bleeding canker of horse chestnut. PDAs were applied preventatively, i.e. before Pae inoculation of plants and therapeutically, i.e. after Pae inoculation of plants and as both as a foliar spray (FS) and root drench (RD). Irrespective of mode of application (i.e. FS, RD) or whether applied preventatively or therapeutically, PDAs containing salicylic acid analogues, plant hormone–vitamin complex and harpin protein possessed limited efficacy as Pae protectant and/or eradication compounds. Only application of potassium and silicon phosphite induced positive effects on plant vitality [increased leaf chlorophyll content, leaf chlorophyll fluorescence (Fv/Fm)], enhanced defensive enzyme activity (β-1,3-glucanase, peroxidase) and reduced Pae lesion size, the main proxy of Pae success or aggressiveness. Preventative rather than therapeutic phosphite application resulted in greater reductions in Pae severity. Little significance of mode of application was demonstrated, indicating that phosphites can be applied as either a FS or RD resulting in similar degrees of Pae control. Significant reductions in Pae severity recorded in this study give credence to the potential of phosphites as an alternative or complement conventional bactericides for Pae control.