Evaluation of a Small-Molecule Compound, N-Acetylcysteine, for the Management of Bacterial Spot of Tomato Caused by Copper-Resistant <i>Xanthomonas perforans</i>

Abstract

Bacterial spot caused by <i>Xanthomonas</i> spp. is one of the major diseases in tomato. <i>Xanthomonas perforans</i> is the main pathogen of bacterial spot on tomato in Florida. Currently, application of copper fungicides is the primary measure used to manage this disease. However, the development of copper resistance in <i>X. perforans</i> and accumulation of copper in the environment are major concerns for excessive use of copper-based products in agriculture. Due to its antibacterial properties and low environmental impact, N-acetylcysteine (NAC), a small molecule commonly used in medicine for human bacterial diseases, has been studied in agriculture for the control of plant bacterial pathogens, including <i>X. citri</i> and <i>Xylella fastidiosa</i>. This study evaluated the effect of NAC alone and in combination with copper on a copper-resistant <i>X. perforans</i> strain in vitro and its ability to control bacterial spot of tomato under greenhouse and field conditions. In vitro, the minimum inhibitory concentration of NAC against the <i>X. perforans</i> strain was 2,048 mg liter^-1. NAC increased sensitivity of the copper-resistant <i>X. perforans</i> to copper in vitro when application of NAC was followed by copper application after 6 h. In greenhouse assays, NAC applied alone or in combination with copper significantly (<i>P</i> < 0.05) reduced the disease severity of bacterial spot on tomato compared with the untreated control. NAC at 100 mg liter^-1 + copper at 300 mg liter^-1 consistently exhibited synergistic effects against bacterial spot. In the field trials, NAC at 1,000 mg liter^-1 + copper at 150 mg liter^-1 significantly reduced disease severity compared with the untreated control. Results from this study demonstrated that NAC significantly reduced the disease severity of bacterial spot of tomato and enhanced the efficacy of copper against copper-resistant <i>X. perforans</i>, indicating that NAC could be applied for the effective management of bacterial spot of tomato.