Exploring antibacterial capacity of different bacterial and fungal strains against Xylella fastidiosa complex

Xylella fastidiosa (Xf) is a xylem-inhabiting plant pathogenic bacterium classified as a quarantine organism in Europe. It infects nearly 700 plant species across 88 families, acting either as a pathogen or an endophyte, and poses a major threat to Mediterranean agriculture. In the Apulia region of Italy three subspecies - X. fastidiosa subsp. pauca (Xfp), multiplex (Xfm), and fastidiosa (Xff) - severely affects olive, almond and grapevine crop, in the order. The lack of effective curative treatments, along with the pathogen’s complex epidemiology, are critical challenges for its control. This study aimed to identify antagonistic microorganisms with potential to be biocontrol agents against X. fastidiosa complex. In vitro dual culture assays showed that 9 out of 51 bacterial isolates, belonging to the genera Pantoea, Bacillus, and Pseudomonas, were able to inhibit Xfp growth, forming clear inhibition zones ranging from 2.0 to 12 mm. Additionally, the antimicrobial activity of the culture filtrates of Aphanocladium album, Clonostachys rosea, Trichoderma atroviride, and Trichoderma koningiopsis and of a bacterial lysate obtained from Bacillus amyloliquefaciens strain D747, was assessed. Fungal filtrates inhibited all three subspecies, with inhibition areas ranging from 20 to 29 mm. The Bacillus lysate also showed strong antimicrobial activity in well diffusion assays, with halo sizes varying by subspecies. These results highlight the potential of selected microbial strains and their metabolites as components of sustainable biocontrol strategies for managing X. fastidiosa and other xylem-associated pathogens in key Mediterranean crops.