Microbial biodiversity in Tunisian olive grove soils: a reservoir of phytopathogenic fungi and potential beneficial microorganisms

Introduction Intercropping in olive orchards increases the risk of soil-borne fungal infections, particularly when associated crops are susceptible to the same pathogens. This study aimed to identify soil-borne microorganisms colonizing the roots and rhizosphere of olive trees in Tunisia intercropped with Solanaceae plants and to evaluate co-occurring bacterial communities for their potential to mitigate wilt disease and promote plant health. Material and methods Endophytic fungi and bacteria were isolated from olive soils and roots collected from three olive orchards subjected to different intercropping systems. Fungal strains were molecularly identified at the species level using Internal Transcribed Spacer (ITS) and translation elongation factor 1-α ( TEF1 ) gene sequencing, while bacterial strains were characterized by rep-PCR profiling and 16S rDNA sequencing. The pathogenicity of selected Fusarium strains was assessed by in vitro inoculation of detached olive leaves, olive twigs, and tomato seedlings. Antagonistic activity of bacterial strains against selected Fusarium species was evaluated using dual-culture assays, and bacteria–fungi interactions were further investigated by scanning electron microscopy (SEM). Results and discussion A total of 83 fungal and 40 bacterial strains were isolated. The fungal community was dominated by Fusarium species (62%), followed by Phoma (13%) and Alternaria (10%) species, while Verticillium dahliae was not detected at any site. The prevalence and virulence of Fusarium varied among olive groves, with the highest incidence observed at Sidi Bou Ali, where olive trees were intercropped with tomato, and the lowest at Kairouan, where potato intercropping was less frequent. Pathogenicity assays showed that 12 out of 15 of the tested Fusarium strains caused symptoms on both olive tissues and tomato seedlings. Bacterial communities were dominated by Bacillus species and Priestia megaterium . Bacillus species were particularly abundant at the site with the highest Fusarium pressure. The in vitro assay showed that several bacteria exhibited antagonistic activity against pathogenic fungi, with growth inhibition ranging from 8% to 68%, including volatile organic compound–mediated effects. SEM analyses revealed that Bacillus amyloliquefaciens inhibited fungal growth through biofilm formation and hyphal alteration.