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.