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Xanthomonas campestris pv. campestris is the causal agent of black rot in crucifers, a plant disease with significant economic impact. Xanthomonadaceae is a large family of Gram-negative bacteria that cause symptoms by blocking water flow in plants by invading the xylem. To accomplish this, the main mechanism the bacteria use to adapt to environmental changes and colonize tissues is biofilm formation. In recent years, growing interest in natural antimicrobial compounds has led to the study of different phytocomplexes derived from plants. In this work, Moringa oleifera was selected, as its leaves are rich in phenols, essential oils, and vitamins that exert antibacterial activity. X. campestris pv. campestris biofilm, one of its major virulence factors, was studied. Biofilm formation and removal were analyzed on abiotic and biotic surfaces with and without M. oleifera leaf extracts. The data from the analysis show that Moringa oleifera leaf extracts and single phenols were able to inhibit biofilm growth on abiotic surfaces, but the activity of the whole phytocomplex was significantly higher compared to that of individual phenols. The effect of Moringa oleifera extracts on cabbage leaves in vivo was also found to be very important, as scanning electron microscopy showed that treatment with the extracts led to clear unblocking of the xylem, implying many advantages for use in black rot control.

Erwinia amylovora (EA) is a phytopathogenic bacterium, the causative agent of bacterial fire blight, a disease that affects Rosaceaes. In order to replace antibiotics and copper, the antimicrobial activity of three extracts of Moringa oleifera Lam., methanolic (MeOH-MOE), hydroalcoholic (HA-MOE) and hydroalcoholic with maltodextrins (HAMD-MOE), was tested on eleven strains of EA isolated from apple trees by the Emilia-Romagna Phytosanitary Department. MIC and MBC have been evaluated; biofilm formation, swarming motility and amylovoran production were performed with the crystalviolet, soft-agar assay and the amylovoran method. All extracts demonstrated bacteriostatic activity at a concentration of 1 mg/mL, resulting in a 80% reduction in biofilm formation. HAMD-MOE, MeOH-MOE and HA-MOE caused an inhibition of motility of 60%, 65% and 30% after 6 days and a decrease in amylovoran synthesis of 84%, 63% and 93%, respectively. In planta results showed how the compounds were able to inhibit EA virulence on apple trees, mainly if they were applied as a preventive treatment, although the treatment showed a significant reduction in fire blight symptoms progression. The antibacterial activity of the extracts is mainly due to the high concentration of polyphenolic compounds detected in the extracts that was able to alter the permeability of bacterial membrane, resulting in slowing the synthesis of ATP and consequently of all ATP-dependent functions, such as motility and less selectivity towards harmful compounds, which can, thus, enter the cytoplasm and inhibit enzymes involved in replication and quorum sensing. The efficacy, eco-compatibility and low cost make such extracts a potential tool for the control of bacterial fire blight.