Native multifunctional Metarhizium spp. delivered via seed coatings suppress soil-borne phytopathogens and preserve rhizobial symbiosis in soybean

Soybean (Glycine max L.) is a leading crop occupying more than one million hectares in Uruguay. Seedlings are affected by fungal diseases and soil-dwelling insects, which together reduce emergence by 15–40%. Accordingly, seed treatments rely on fungicides and insecticides that affect the viability of Bradyrhizobium elkanii inoculants, reducing biological nitrogen fixation and leaving environmental residues. Entomopathogenic Metarhizium spp. infect insects and antagonise phytopathogens, promoting plant performance. Native Metarhizium isolates were screened in vitro and in planta for compatibility with commercial B. elkanii strains U1301 and U1302, and against Fusarium graminearum, Pythium sylvaticum, Pythium ultimum and Rhizoctonia oryzae. Although several isolates caused inhibition around rhizobial colonies in vitro, seed-coating trials showed no impairment of nodulation or shoot biomass, which confirms compatibility. Metarhizium robertsii ILB167 and ILB440 persisted epiphytically (102–103 CFU/g); but no endophytic colonisation was detected. Dual-culture assays identified M. robertsii ILB167 and ILB440 as the most effective inhibitors, reducing phytopathogen growth to 53%. These isolates, together with M. brunneum ILB429, produced the widest inhibition halos, indicating diffusible antifungal metabolites. In naturally infested soil, the M. robertsii isolates increased seed germination by more than 10%. Under controlled pathogen pressure, M. robertsii ILB440 improved seedling emergence in the presence of F. graminearum and R. oryzae, whereas no isolate alleviated Pythium spp. losses. Solid-substrate fermentation on rice grains yielded up to 17.2 × 108 conidia/g for M. robertsii ILB440. Collectively, M. robertsii ILB440 is a robust candidate for integrated seed coatings combining biocontrol, rhizobial compatibility and scalable production, aligning with Uruguay’s National Plan for Bioinputs.