Biocontrol Potential of Bacillus subtilis AM-7 (RSP2) against Sheath Blight of Rice (Oryza sativa) caused by Rhizoctonia solani.

Abstract

Rice sheath blight, a serious global threat to rice production, is caused by the soil-borne fungal pathogen Rhizoctonia solani. A major focus in sustainable agriculture is the use of beneficial microorganisms to enhance plant health and suppress disease. This study aims to identify a potent biocontrol agent against R. solani while simultaneously promoting plant growth. After isolating 85 rhizobacterial strains from the rhizospheric soil of Bhandara district associated with Jayshreerm, Parmal, and Wada varieties of Oryza sativa (rice), they were screened against Rhizoctonia solani using the dual culture method. Among them, strain AM-7 (RSP2) emerged as the most effective, exhibiting a maximum inhibition zone of 42 mm and an inhibition rate of 40%. Twenty potential isolates were selected based on their biochemical characteristics. These isolates were further evaluated for their plant growth-promoting rhizobacteria (PGPR) activities, including the production of hydrogen cyanide (HCN), ammonia, siderophores, indole-3-acetic acid (IAA), phosphate, zinc, and potassium solubilization, nitrogen fixation, and chitinase activity. In vitro screening against Rhizoctonia solani was conducted using the dual culture method. Among these, isolate AM-7 was identified as Bacillus subtilis strain RSP2 through 16S rRNA gene sequencing. Volatile organic compounds (VOCs) produced by B. subtilis were found to be effective in suppressing Rhizoctonia solani in vitro. Fourier- transform infrared (FTIR) analysis of the crude extract of isolate AM-7 (RSP2) revealed the presence of functional groups such as an alkyne group (3308.31 cm⁻¹), amine salt (2942.79 cm⁻¹), carboxylic acid group (2831.73 cm⁻¹), and secondary alcohol (1114.95 cm⁻¹). All isolates tested positive for indole-3-acetic acid (IAA) production; however, only isolate AM-7 was found to produce chitinase. Upon optimization, the maximum IAA production by isolate AM-7 was recorded as 175 μg/ml after 48 hours of incubation at 37 °C. Sheath blight control is challenging due to the pathogen's adaptability, absence of resistant rice varieties and genes, and limited farmer awareness.