Field Performance of Tailored Bacterial-Fungal Consortia Against Rhizoctonia solani in Lettuce

Summary

Soil-borne fungal pathogens are a major threat to high-value crops such as vegetables. Although synthetic fungicides remain the most effective control strategy, their intensive use threatens ecosystem functioning and long-term sustainability. Microbial biocontrol agents offer a promising alternative, but their effectiveness in the field is often inconsistent and requires improvement. This study aimed to enhance biocontrol efficacy by combining beneficial fungi and bacteria into tailored microbial consortia. We hypothesized that synergistic interactions, such as fungal highways, could improve establishment and pathogen suppression. Further, we are developing a novel formulation based on encapsulation of the microorganisms. Pathosystem-specific consortia were developed to support lettuce against the soil-borne pathogen Rhizoctonia solani. The fungus Clonostachys rosea was combined with either Comomonas testosteroni or Pseudomonas entomophila, all previously isolated from Swiss disease-suppressive soils. A second strain of C. rosea and Bacillus amyloliquefaciens, derived from the commercial products Prestop® and Amylo-X®, served as positive controls. Three field trials were conducted in autumn 2025. Two agronomic and three microbial assessments were performed to evaluate plant performance and microbial establishment. Agronomic measurements revealed neither increased biomass nor reduced disease severity in treated plots. Ongoing microbial analyses will clarify whether adjustments in inoculation method, concentration of applied microorganisms, or consortium composition are required to improve field efficacy.