Evaluation of drench treatments with phosphonate derivatives against pseudomonas syringae pv. syringae on pear under controlled environment conditions

Several phosphonate derivatives including the oomycetic antifungal agents phosphonate and tris-o-ethylphosphonate (fosetyl), the ethylene-releasing compound 2-chloroethylphosphonate (ethephon), and the antibiotic 2-epoxypropylphosphonate (phosphomycin) were evaluated for in vitro and in planta activity against Pseudomonas syringae pv. syringae. Inhibition of colony growth in CYE agar by phosphonate, fosetyl and etephon was very slight (minimal inhibitory concentrations MIC= 0.31-0.62 g HPO₃^2-/l). Also, survival of P. syringae pv. syringae in aqueous solutions of phosphonate or fosetyl was high. Only phosphomycin showed significant antibacterial activity in vitro (MIC=10-20 μg HPO₃^2-/ml) compared to streptomycin (1-2 μg a.i./ml). Potted pear plants irrigated with these chemicals and inoculated with Pseudomonas syringae pv. syringae had significantly less disease than non-treated controls (P<0.001). Phosphomycin was the most active compound with a median effective dose (ED₅₀) of less than 0.62 g HPO₃^2-/l. Activities of the other phosphonates were weak but consistent between experiments. The ED₅₀s on whole plants were 2.1, 3.3, and 6.9 g HPO₃^2-/l for ethephon, phosphonate and fosetyl, respectively. The ED₅₀ of P. syringae pv. syringae increased from 6.5 in non-treated controls to 7.7-8.8 log₁₀ cfu/ml on plants treated with phosphonates at 1.86 g HPO₃^2-/l. It was concluded that drench treatment with fosetyl is not a practical option for control of P. syringae pv. syringae on pear.