The plant-pathogenic bacteriumXanthomonas citrisubsp.citriis the causal agent of Asiatic citrus canker, a serious disease that affects all the cultivars of citrus in subtropical citrus-producing areas worldwide. There is no curative treatment for citrus canker; thus, the eradication of infected plants constitutes the only effective control of the spread ofX. citrisubsp.citri. Since the eradication program in the state of São Paulo, Brazil, is under threat, there is a clear risk ofX. citrisubsp.citribecoming endemic in the main orange-producing area in the world. Here we evaluated the potential use of alkyl gallates to preventX. citrisubsp.citrigrowth. These esters displayed a potent anti-X. citrisubsp.citriactivity similar to that of kanamycin (positive control), as evaluated by the resazurin microtiter assay (REMA). The treatment ofX. citrisubsp.citricells with these compounds induced altered cell morphology, and investigations of the possible intracellular targets usingX. citrisubsp.citristrains labeled for the septum and centromere pointed to a common target involved in chromosome segregation and cell division. Finally, the artificial inoculation of citrus withX. citrisubsp.citricells pretreated with alkyl gallates showed that the bacterium loses the ability to colonize its host, which indicates the potential of these esters to protect citrus plants againstX. citrisubsp.citriinfection.

Methodology for testing natural compounds for determination of antifungal activity had been developed with adaptations. The most used are bioautography and agar diffusion with a complex and no defined media. In this study, different methods for determination of antifungal activity of natural products are discussed and the use of M27-A2 microdilution test from CLSI (Clinical and Laboratory Standards Institute, 2002), as general standard methodology for testing plant extracts activity is recommended.