M. Young, A. Ozcan, M.E. Myers, J.H. Graham, S. Santra
University of Central Florida,
Keywords: core-shell, copper, silica particle, antimicrobial, silica gel, citrus canker
Summary:Copper (Cu) bactericides/fungicides are used extensively for crop protection in agriculture. There is an increasing concern for Cu accumulation in fertile soil and potential for Cu leaching into the surrounding ecosystem. Moreover, development of Cu resistance in phytopathogenic bacteria is widespread. While there is no suitable alternative to Cu available to date for agricultural uses, there is potential to improve efficacy through engineering of Cu material’s structure and its environment. Most commercial Cu products (Cu oxides, Cu hydroxides, Cu oxychlorides) are low in water solubility and risk of phytotoxicity because Cu bioavailability is limited. Cu bioavailability of water-soluble Cu compounds (Cu salts and Cu chelates) is higher, however, they may cause phytotoxicity. To address these limitations, we developed a core-shell-Cu particles (CS-CuPs) material. In the CS-CuP system, an inert particulate silica core is coated with a shell of Cu loaded silica nanogel to form the C-S CuP. XPS studies showed that Cu is present in the silica matrix in multiple valence states (0, +1 and +2) states. CS-CuPs were characterized by high-resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM), to determine particle size and morphology, crystallinity, and Cu loading. Electron microscopy revealed that CS-CuPs are sub-micron in size and spherical in shape. Phytotoxicity studies were performed in Vinca sp. and sour orange citrus under greenhouse conditions. Results showed that CS-CuPs treatments are safe to plants, demonstrating potential usefulness as an agricultural biocide. Effectiveness of CS-CuPs in-vitro was evaluated using model plant bacterial species including the plant pathogen, Xanthomonas alfalfae subsp. citrumelonis. The MIC for the CS-CuPs was found to be lower than the copper hydroxide control. To prove the potential of CS-CuPs under field conditions, trials were conducted during the 2014 citrus season in Ft. Pierce, Florida, USA on a 7 year-old Ray Ruby grapefruit. Analysis of citrus canker caused by Xanthomonas citri subsp. citri revealed a 62.8% incidence of fruit lesions on untreated trees while sprays of commercial copper hydroxide reduced incidence to 16.4 % when applied at a rate of 0.9 lb metallic Cu/acre. In comparison, CS-CuPs reduced canker incidence to 15.6 % at a rate of 0.2 lb metallic Cu/acre. Crop protection exhibited by CS-CuPs at a significantly lower metallic Cu rate demonstrates the high potential for this material as an agricultural bactericide/fungicide.