In vitro screening of biological control agents and antimicrobial potential of plant extracts against Xylella fastidiosa subsp. pauca

Xylella fastidiosa is a devastating pathogen that threatens economically significant crops, such as olive trees. Currently, effective control methods for this bacterium remain elusive. In this context, screening for antimicrobial activity using various plant extracts and biological control agents may offer insights into novel plant protection products that could be integrated into control strategies for xylem-colonizing pathogens, such as Xylella fastidiosa. Antimicrobial metabolites produced by antagonistic microorganisms play a crucial role in managing plant diseases. While natural antagonists have traditionally been employed for disease control, the use of microbial metabolites offers several advantages over utilizing entire microorganisms. In this study, in vitro disc diffusion and broth dilution assays were performed to evaluate the antimicrobial activity of various plant extracts against Xylella fastidiosa subsp. pauca (Xfp). Additionally, the antagonistic activity of various Bacillus species was screened using a dual culture assay. Our results demonstrated significant antimicrobial effects of carob extract and different halophytes, such as Suaeda maritima. A clear inhibition zone of 5.7 mm was observed for the carob extract. Additionally, Suaeda extract effectively reduced Xfp growth (OD600 = 0.45 after 192 hours), compared to the control (OD600 = 0.54 after 192 hours). Interestingly, the in vitro dual culture test showed that some Bacillus isolates inhibited X. fastidiosa growth, with a clear inhibition zone of 5.4 mm. Further investigation into the potential of these extracts and antagonistic bacteria to manage X. fastidiosa is warranted, as their secreted compounds could lead to the development of a sustainable control strategy.