Evaluation of a small molecule compound 3-indolylacetonitrile for control of bacterial spot on tomato

Bacterial spot, caused by Xanthomonas perforans, is one of the major diseases in fresh market tomato production in Florida. It is reported that X. perforans has developed resistance to copper, an element of major bactericides used for management of this disease in Florida and worldwide. Continuous efforts are necessary for providing the tomato industry with new effective approaches to managing this important disease. A small molecule 3-indolylacetonitrile (IAN) that can inhibit biofilm formation has been reported to be effective against many medical bacterial pathogens. In this study, we evaluated the efficacy of IAN in planta for the control of bacterial leaf spot on tomato, the effectiveness of IAN when applied at different times from pathogen inoculation, how IAN efficacy compared to a copper bactericide (Kocide 3000), and if IAN could be applied as a tank mixture with copper bactericide. In vitro testing in liquid media was also completed to further investigate observations made in planta. Our results showed that IAN was effective in reducing disease severity of bacterial leaf spot on tomato between 0.5 and 1.0 mg/ml. However, phytotoxicity to tomato plants occurred when the concentration was higher than 1.0 mg/ml. IAN significantly (P < 0.05) reduced the disease when it was foliar applied during the period from 3 days prior to inoculation to 1 day after inoculation. When IAN was applied in combination with a copper bactericide Kocide 3000, additive effect was observed in reducing both disease severity and bacterial growth, compared to either IAN or copper alone, for both copper-sensitive and copper-resistant X. perforans strains. Results from this study indicated that IAN significantly reduced bacterial leaf spot on tomato and enhanced the efficacy of copper against X. perforans. Specifically, IAN significantly improved the control of bacterial leaf spot caused by a copper-resistant X. perforans strain.