The role of Bacillus marisflavi in enhancing wheat crown rot through Fusarium pseudograminearum activation.

While many root-associated bacteria promote plant growth or induce resistance, Bacillus marisflavi interaction enhanced wheat crown rot (WCR) caused by Fusarium pseudograminearum. Unlike typical beneficial Bacillus species, B. marisflavi did not directly affect wheat growth but significantly increased pathogen colonization of roots and stem bases, exacerbating WCR incidence. This occurred via activation of F. pseudograminearum genes linked to growth and pathogenicity. The B. marisflavi metabolite D-mannitol promoted proliferation of F. pseudograminearum and related Fusarium spp., elevated mycotoxin (deoxynivalenol, DON) production, and potentially shielded pathogens against plant reactive oxygen species (ROS) defenses. This contrasts with the typical biocontrol role of Bacillus. Our findings reveal that previously overlooked soil bacteria can facilitate fungal disease within the same niche, even without direct plant effects. This highlights that neglecting specific microbes or their metabolites may contribute to the limited field efficacy of antagonistic biocontrol agents targeting pathogens.