Biocontrol Potential of Bacillus Strains from Grapevine Rhizosphere Against Allorhizobium vitis, Causal Agent of Crown Gall Disease in Moroccan Vineyards

Beneficial bacteria are recognised for their antimicrobial compounds, making them valuable for disease control in agriculture. Bacillus species stand out for their stability, versatility, and selectivity as biocontrol agents. This study aimed to identify potential antagonists within the rhizosphere microorganisms by isolating bacterial strains from grapevine roots and rhizosphere soil in Moroccan vineyards. The antimicrobial activities of these isolates against Allorhizobium vitis, the causative agent of grapevine crown gall, were evaluated in vitro using a disc diffusion assay, followed by in planta assessments under preventive and simulated inoculation conditions. Screening led to the isolation of 123 strains, with six showing strong antagonistic properties, achieving inhibition percentages up to 39.6%. 16S rRNA sequencing led to identifying five Bacillus species: B. amyloliquefaciens, B. velezensis, B. halotolerans, B. subtilis, and B. anthracis. These strains were further characterised by their biochemical traits and plant growth-promoting abilities. Compatibility assays identified optimal combinations for microbial consortia, demonstrating pathogen inhibition up to 37.4%. In planta bioassays confirmed the effectiveness of the isolates and consortia, reducing tumour size. These findings highlight the potential of these Bacillus strains as biocontrol agents and underscore the value of microbial consortia as a sustainable approach to managing grapevine crown gall.