Antibacterial activity of Myrtus communis leaf extract against three pathogenic Xylella fastidiosa subspecies

The limited availability of sustainable strategies for managing <i>Xylella fastidiosa</i> (Xf) underscores the urgent need for innovative and practical antimicrobial solutions. In this study, an extract from the leaves of <i>Myrtus communis</i> (MC) biotype "Tarantino", known for its antibacterial properties, was evaluated <i>in vitro</i> using well diffusion and broth dilution assays against <i>X. fastidiosa</i> subsp. <i>pauca</i> (Xfp), <i>multiplex</i> (Xfm), and <i>fastidiosa</i> (Xff), all of which have been reported in Europe and in the Apulia region. Prior to biological testing, the MC leaf extract (MCLE) underwent chemical characterization via LC-Q-Exactive-Orbitrap/MS and LC-Q-Exactive-Orbitrap/MS/MS. Metabolite profiling revealed the presence of several phenolic acids, flavonol derivatives, and ellagitannins. In the <i>in vitro</i> assays, MCLE exhibited clear inhibition zones against all three Xf subspecies, with diameters ranging from 4.8 to 22.6 mm compared to the control. Additionally, in broth cultures, a 1:5 dilution of MCLE significantly inhibited bacterial growth, resulting in 65%, 86%, and 66% inhibition for Xfp, Xfm, and Xff, respectively. For <i>in planta</i> assays, symptom severity on the canopy of <i>Nicotiana benthamiana</i> was notably reduced in plants treated with MCLE compared to those inoculated with Xf alone. Quantitative real-time PCR confirmed the efficacy of MCLE: untreated, inoculated plants exhibited significantly lower Cq values (F = 120; p < 0.001), indicating higher bacterial loads compared to MCLE-treated plants. Overall, this study highlights the potential of MC-based formulations as promising, eco-friendly tools for the management of Xf-related diseases, meriting further validation under field conditions.