Synergistic effect of Cordia curassavica Jacq. essential oils association against the phytopathogen Xanthomonas campestris pv. campestris.

The increased use of pesticides applied to treat diseases caused by bacteria has caused serious environmental problems. There are few fungicides/bactericides for the treatment of plant diseases caused by Xanthomonas campestris pv. campestris (Xcc), and only two natural products with general bactericidal/fungicidal use are available on the market. Thus, this study evaluated the antimicrobial activity of essential oils (EOs), and their combinations, from five distinct genotypes of Cordia curassavica (Jacq.) Roem. & Schult (Syn. Varronia curassavica Jacq.) (CCUR) against Xcc. GC/MS chemical analysis revealed α-pinene, sabinene, (E)-caryophyllene, ar-curcumene, ß-sesquiphellandrene, 7-cyclodecen-1-one, and ar-Turmerone as the major compounds of the five EOs of CCUR. All EOs showed growth inhibition of Xcc with minimum inhibitory concentration between 500 and 1000 µg mL-1. The associations between two EOs from different CCUR genotypes showed that 70% of the total combinations had an additive effect. However, the combinations between CCUR-002 × (-302, -202) and CCUR-302 × (-601) showed a synergistic effect, with mean fractional inhibitory concentration FIC50 values of 0.28, 0.42, and 0.40, respectively. This study demonstrates that combinations of C. curassavica EOs have antimicrobial activity and a potential to be used in the control of black rot. Graphical abstract.

Antimicrobial activity of Lippia gracilis essential oils on the plant pathogen Xanthomonas campestris pv. campestris and their effect on membrane integrity.

Xanthomonas campestris pv.campestris (Xcc) is the causative agent of black rot, a disease that causes serious damage to plants from Brassicaceae family. However, there are no chemicals or biological agent commercially registered for the control of this disease. Thus, this study aimed to evaluate the antimicrobial activity and chemical composition of Lippia gracilis essential oils (EOs) on Xcc aiming its use as effective biological control. We also investigated the effect of EOs on the integrity of the bacterial cytoplasmic membrane. Chemical analysis by GC/MS showed that the major compounds of the seven EOs of L. gracilis are thymol or carvacrol. The seven genotypes showed inhibition of bacterial growth with MIC from 700 µg.ml-1 to 1000 µg.ml-1, with the genotype LGRA-106 (rich in Thymol) with higher antimicrobial activity. The MIC for thymol and carvacrol were 250 µg.ml-1. After exposure to LGRA-106 EO (2×, 1×, 1/2×, 1/4×, and 1/8 x MIC for 5 min, it was observed a decreased cell viability and increased pI fluorescence, which indicates damage to the cytoplasmic cell membrane. This study demonstrates that L. gracilis EOs have antimicrobial activity and have a potential to be used in the control of black rot. Furthermore this antimicrobial activity is due, at least in part, to bacterial cytoplasmic membrane damage.