Biocontrol and growth-promoting potential of Bacillus subtilis against root-rot disease in sugar beet

Abstract

Among the various pathogens that cause sugar beet root rot, Rhizoctonia solani is one of the most destructive species globally. This pathogen causes damping-off and root rot during field growth and storage, leading to crop yield losses; therefore, the development of environmentally friendly prevention and control strategies targeting this organism is urgently needed. This study aimed to evaluate, through laboratory and greenhouse experiments, the biological control potential of Bacillus subtilis against the sugar beet root rot pathogen R. solani, and to investigate their plant growth-promoting properties. Pathogenicity tests revealed that all tested fungal isolates were capable of infecting sugar beet plants and causing root rot symptoms. Among them, isolate KA 3 showed the highest percentage of damping-off and root rot severity. Therefore, the most aggressive isolate was selected for subsequent antagonistic and greenhouse experiments. This study employed the dual culture method for in vitro screening. The 15 Bacillus strains isolated from the sugar beet rhizosphere exhibited varying levels of antagonistic activity. Among them, B. subtilis strain K 2 had the strongest activity, with a mycelia inhibition rate of 62.51% against R. solani. Under greenhouse conditions, B. subtilis isolate K2 reduced disease severity from 95.5% in the infected control to 22.4%, corresponding to 76.5% disease suppression, while isolate K5 reduced disease severity to 32.3% (66.2% suppression). In addition, treatment with B. subtilis can significantly promote the growth of infected plants, increasing fresh, dry, root, and root dry weight of sugar beet plants. Furthermore, B. subtilis treatment significantly increased total phenolic and flavonoid contents in sugar beet plants. This study proposes that Bacillus subtilis is a promising biocontrol agent and plant growth promoter, which can be incorporated into integrated disease management (IDM) programs for the sustainable prevention and control of sugar beet root rot.