Silver Nanoparticles Suppress Apple Fire Blight: Antibacterial Activity, Reduced Carbohydrate-Containing EPS Content, and Altered Host Antioxidant Responses

The present study evaluated the phytoprotective activity of silver nanoparticles stabilized with sodium amphopolycarboxyglycinate (AgNPs-APCG) against apple fire blight caused by Erwinia amylovora. A highly colloidally stable AgNPs-APCG dispersion was prepared by borohydride chemical reduction and contained predominantly spherical metallic AgNPs with an average diameter of 10.4 ± 4.9 nm and a ζ-potential of −52.6 ± 2.4 mV. The obtained dispersion showed pronounced antibacterial activity against the E. amylovora strain, isolated from fire blight-affected shoots, with a MIC of 18.75 mg/L. In the field trial, foliar application at 5 and 25 mg/L reduced fire blight incidence and severity, with biological efficacies of 52.8% and 80.3%, respectively, and increased marketable yield by 12.0% and 16.7%. The phytoprotective efficacy of AgNPs-APCG at 5 mg/L, a concentration nominally below the in vitro MIC for E. amylovora, was consistent with a possible contribution of factors in addition to direct antibacterial activity. Supporting this possibility, AgNPs-APCG reduced carbohydrate-containing exopolysaccharide content in E. amylovora cultures to 51 ± 5% of the untreated control. Furthermore, higher activities of antioxidant enzymes were observed in AgNPs-APCG-treated apple leaves, while malondialdehyde content showed no pronounced increase, suggesting possible modulation of redox- and defense-related metabolism in treated plants. Potential environmental implications of the tested application regimes were considered based on the estimated Ag input to soil. These findings suggest that multiple pathogen- and host-associated responses may contribute to the phytoprotective effect of AgNPs-APCG and support its further evaluation as a multifunctional agent against bacterial plant diseases.