Fixed-Quat: An Attractive Nonmetal Alternative to Copper Biocides against Plant Pathogens

In this paper, we report a nonphytotoxic bactericide and fungicide formulation containing a composite of silica and quaternary ammonium compound (quat). The composite material was prepared using an acid-catalyzed sol-gel method. Positively charged quat was associated with a negatively charged silica-gel matrix, producing a stable suspension of fixed-quat gel (FQ-G). The morphology of FQ-G and the interaction of quat with silica were characterized using SEM and FTIR, respectively. Silica gel significantly reduced quat phytotoxicity when tested at 500 and 1000 μg/mL foliar-application rates. The in vitro antimicrobial efficacy of FQ-G was evaluated against Xanthomonas alfalfae, Pseudomonas syringae, and Clavibacter michiganensis, showing comparable efficacies to that of quat itself. In field conditions, its efficacy in controlling the bacterial and fungal diseases citrus canker, scab, and melanose on 'Ray Ruby' red grapefruit was evaluated. Foliar application rates at 100 and 200 μg/mL provided comparable disease control to those of several copper standards, demonstrating the potential for use as an alternative agricultural biocide.