Management of bacterial spot of tomato caused by copper-resistant Xanthomonas perforans using a small molecule compound carvacrol.
Bacterial spot of tomato (BST), caused by Xanthomonas perforans, is one of the most serious diseases in fresh market tomato production in Florida, and currently available management measures are largely ineffective or insufficient. Using small molecule compounds has been shown to be promising in management of BST. Carvacrol is a plant-derived small molecule, which is effective against many human and plant pathogens. In this study, we evaluated the efficacy of carvacrol for control of BST in vitro, in the greenhouse and in field trials. In the laboratory, X. perforans strain used in this study was able to grow on nutrient agar (NA) amended with CuSO4 at 400 mg L-1, which proved that this X. perforans strain was copper resistant. Our results also showed that the minimum inhibitory concentration (MIC) of carvacrol was 32 mg L-1 and its minimum bactericidal concentration (MBC) was 64 mg L-1. Carvacrol increased the sensitivity of the copper-resistant X. perforans strain to copper. In the leaf disc assay, carvacrol combined with copper reduced recovery of X. perforans from tomato leaf discs. Seed priming with carvacrol at 32 mg L-1 was found to be effective in promoting seed germination and seedling vigor of tomato. In greenhouse assays, carvacrol foliar applied alone or in combination with copper significantly (P < 0.05) reduced BST severity. Moreover, carvacrol at 32 mg L-1 combined with copper at 76.8 mg L-1 was most effective in reducing BST severity, which was significantly greater than either treatment alone and also greater than copper at 153.6 mg L-1. In field trials during 2018 and 2019, control efficacy of carvacrol at 32 mg L-1 was significantly greater than copper. Results from this study indicate that carvacrol significantly reduces BST severity and enhances the efficacy of copper against copper resistant X. perforans. There is potential for the small molecule compound carvacrol to be integrated into BST management programs for copper resistant X. perforans and provide a more environmentally sustainable approach for controlling BST.