Citrus canker caused by the bacterium Xanthomonas citri subsp. citri (Xac) is an economically important disease of citrus worldwide. Biofilm formation plays an important role in early infection of Xac on host leaves. In this study, we assessed the hypothesis that small molecules able to inhibit biofilm formation reduce Xac infection and enhance the control of citrus canker disease. D-leucine and 3-indolylacetonitrile (IAN) were found to prevent biofilm formation by Xac on different abiotic surfaces and host leaves at a concentration lower than the minimum inhibitory concentration (MIC). Quantitative real time reverse transcription polymerase chain reaction (qRT-PCR) analysis indicated that IAN repressed expression of chemotaxis/motility-related genes in Xac. In laboratory experiments, planktonic and biofilm cells of Xac treated with D-leucine and IAN, either alone or in combination, were more susceptible to copper (CuSO4) than those untreated. In greenhouse assays, D-leucine and IAN applied alone, or combined with copper reduced both the number of canker lesions and bacterial populations of Xac on citrus host leaves. This study provides the basis for the use of foliar-applied biofilm inhibitors for the control of citrus canker alone or combined with copper-based bactericides.
