Zinc Oxide-Based Nanoformulation Zinkicide Mitigates the Xylem-Limited Pathogen <i>Xylella fastidiosa</i> in Tobacco and Southern Highbush Blueberry

Abstract

The xylem-limited pathogen <i>Xylella fastidiosa</i> causes severe economic losses worldwide, and no effective antimicrobial disease management options are available. The goal of this study was to evaluate the efficacy of a novel ZnO-based nanoparticle formulation, Zinkicide TMN110 (ZnK), against <i>X. fastidiosa</i> in vitro and in planta. In vitro, minimum bactericidal concentration (MBC) of ZnK analyzed in Pierce's Disease 2 medium was estimated at approximately 60 ppm. Time-kill kinetics assay showed a 100% reduction of culturable <i>X. fastidiosa</i> in less than 1 h after ZnK treatment. Microfluidic chambers assays showed that ZnK also inhibits <i>X. fastidiosa</i> cell aggregation and growth under flow conditions. Phytotoxicity assessments in the greenhouse demonstrated that ZnK can be applied as a soil drench in 50 ml at 500 ppm/plant/week up to four times to tobacco and blueberry without causing visible damage. ZnK was also evaluated for disease control in the greenhouse using tobacco infected with <i>X. fastidiosa</i> subsp. <i>fastidiosa</i> strain TemeculaL. ZnK soil drench weekly applications at concentrations of 500 followed by 1,000 ppm (500/1,000) and 500/500/1,000 ppm (in 50 ml each), reduced <i>X. fastidiosa</i> populations by >2 to 3 log₁₀ units and disease severity by approximately 57 and 76%, respectively, compared with the untreated control. Similarly, when blueberry plants infected with <i>X. fastidiosa</i> subsp. <i>multiplex</i> strain AlmaEm3 were soil drenched with ZnK at concentrations 1,000/1,000 ppm and 1,000/1,000/500 ppm (in 200 ml each), the bacterial population was reduced by approximately 1 to 2 log₁₀ units, and disease severity decreased by approximately 39 and 43%, respectively. Overall, this study shows antibacterial activity of ZnK against <i>X. fastidiosa</i> and its effectiveness in plants to reduce disease symptoms under controlled conditions.