Soil microbial diversity: A key factor in pathogen suppression and inoculant performance

Soil microbial diversity plays a crucial role in plant health, influencing pathogen suppression and biocontrol efficacy. This study investigated how soil microbial diversity modulates interactions between the pathogen <i>Bipolaris sorokiniana</i> and the biocontrol bacterium <i>Pseudomonas inefficax</i> in the wheat rhizosphere. Using a dilution-to-extinction method, we established five soil microbial diversity levels: natural soil, dilutions at 10^-1, 10^-3, 10^-6, and fully autoclaved soil. This gradient allowed us to evaluate disease severity, plant growth, and rhizosphere microbiome shifts. Inoculation with <i>Pseudomonas inefficax</i> significantly reduced disease severity caused by <i>Bipolaris sorokiniana</i>, particularly in low-diversity soils, emphasizing the effectiveness of <i>P. inefficax</i> in these simplified environments where microbial competition is reduced. Despite higher pathogen abundance in low-diversity soils, <i>P. inefficax</i> effectively mitigated disease severity, likely through direct antagonistic activity. Alpha diversity indices confirmed a reduction in microbial diversity across the gradient, while beta diversity analyses revealed distinct shifts among treatments. Although <i>Chitinophaga</i>, <i>Pseudomonas</i> and <i>Dyadobacter</i> were significantly enriched in natural soils with inoculation of the <i>P. inefficax</i>, statistically significant disease suppression was not observed under these higher-diversity conditions. On the other hand, in low-diverse soils (autoclaved soil), where disease is suppressed with <i>P. inefficax</i> inoculation, <i>Fluviicola</i> showed a significant enrichment when compared with the treatment inoculated only with the pathogen, suggesting that this bacterial taxon can play a role in disease suppression along with the inoculant. These findings underscore the critical role of the soil microbial diversity in shaping the success of biocontrol interventions.