Low Concentrations of a Silver-Based Nanocomposite to Manage Bacterial Spot of Tomato in the Greenhouse

Bacterial spot, caused by four Xanthomonas spp., is one of the most damaging diseases of tomato worldwide. Due to limited disease management options, growers rely heavily on copper-based bactericides, which are often ineffective due to the presence of copper-resistant Xanthomonas strains. This study was undertaken to characterize the antibacterial activity of a silver-based nanocomposite, Ag-dsDNA-GO, and its potential as an alternative to copper. Ag-dsDNA-GO at rates as low as 10 mu g/ml killed all bacterial cells of copper-tolerant and-sensitive Xanthomonas perforans strains in suspensions containing approximately 10(3) CFU/ml within 15 min of exposure in vitro, whereas equivalent rates of copper (10, 25, and 50 mu g/ml) were unable to significantly reduce populations compared with the untreated control after 24 h of exposure (P = 0.05). All copper concentrations killed the copper-sensitive X. perforans strain but required exposure for >= 1 h. Ag-dsDNA-GO also exhibited antibacterial activity against copper tolerant X. vesicatoria, X euvesicatoria, and X. gardneri strains. In greenhouse studies, tomato plants treated with Ag-dsDNA-GO at either 75 or 100 mu g/ml prior to artificial inoculation significantly reduced disease severity when compared with copper-mancozeb and negative controls (P = 0.05). This study highlights the potential of Ag-dsDNA-GO as an alternative to copper in tomato transplant production.