Molecular Characterization and Biocontrol Potential of Native Rhizobacteria Against Rhizoctonia solani in Rice

Key findings

• ▸Of 32 native rhizobacterial isolates collected from rice rhizosphere soil across four Karnataka growing regions, the actinobacterial strains GVTAM 8 and DWRAM 10 inhibited R. solani mycelial growth by 90.61% and 88.38% in vitro — outperforming even the established reference biocontrol strain AUDT 502 (87.77%).
• ▸In glasshouse trials, combined seed treatment plus foliar spraying with GVTAM 8 and AUDT 502 was the single best treatment, cutting relative lesion height to 14.52% — statistically on par with the chemical fungicide hexaconazole (14.04%) and a dramatic improvement over the untreated, pathogen-only control (65.40%).
• ▸16S rRNA gene sequencing identified GVTAM 8 as Streptomyces cinnabarinus (98.06% similarity to reference sequences) and DWRAM 10 as Streptomyces pseudogriseolus (98.21% similarity), with both sequences deposited to NCBI (accession numbers OQ512000 and OQ512154).
• ▸The biocontrol effect is attributed to two specific mechanisms: hydrogen cyanide production, which disrupts the pathogen's electron transport chain and halts ATP synthesis, and siderophore production, which starves the fungus of iron it needs to survive by binding available ferric ions first.

Abstract

Sheath blight of rice, caused by Rhizoctonia solani Kuhn, ranks as the second most devastating fungal disease of rice after blast, leading to significant yield and quality losses. Current management strategies rely heavily on chemical fungicides, which pose serious environmental and health risks. In recent years, the use of native microbial antagonists has emerged as a promising, eco-friendly alternative for sustainable disease management. In the present study, 32 native rhizobacterial isolates comprising 30 actinobacteria and 2 fluorescent bacteria were obtained from the rhizosphere of healthy rice plants across major rice-growing regions of Karnataka. These isolates were evaluated for their antagonistic potential against a virulent R. solani isolate (RS4) under both in vitro and in vivo conditions. Among the isolates, the actinobacterial strains GVTAM 8, DWRAM 10 and the reference strain AUDT 502 exhibited significant inhibitory effects of 90.61%, 88.38% and 87.77%, respectively, under in vitro conditions. Subsequent glasshouse experiments concluded that seed treatment followed by foliar spraying with GVTAM 8 and AUDT 502 was most effective in reducing sheath blight disease severity, recording lowest relative lesion height of 14.52%, which was statistically on par with hexaconazole treatment. Molecular identification confirmed GVTAM 8 and DWRAM 10 as Streptomyces cinnabarinus and Streptomyces pseudogriseolus, respectively. The biocontrol activity of these actinobacteria makes them a suitable candidate for inclusion in disease management programs, thereby avoiding the complete dependency on chemicals for the management of sheath blight disease.

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