Biocontrol mechanism of strain DB02 against watermelon wilt and damping-off diseases caused by Fusarium oxysporum f. sp. niveum and Rhizoctonia solani

Watermelon wilt and standing blight, caused by Fusarium oxysporum and Rhizoctonia solani respectively, are major soil-borne diseases that severely affect watermelon cultivation. In this study, we isolated a strain from the watermelon rhizosphere that demonstrated significant biocontrol effects, and this strain was identified as Klebsiella aerogenes DB02. Experimental results showed that DB02 exerted significant inhibitory effects on the mycelial fresh weight and spore production of both F . oxysporum and R . solani , with inhibition rates of 52.19% and 51.03% for mycelial fresh weight and 86.21% and 85.71% for spore production, respectively. In pot experiments, DB02 reduced the disease incidence of the two pathogens by 48.57% and 51.73%, respectively, demonstrating its strong biocontrol potential. To further elucidate the biocontrol mechanism of DB02, we analyzed its extracellular metabolites using gas chromatography-mass spectrometry and identified six volatile organic compounds (VOCs) with significant inhibitory effects on F . oxysporum and R . solani . Meanwhile, intracellular metabolomic profiling revealed changes in the metabolic pathways of DB02 during biocontrol, notably its ability to metabolize tryptophan into indole, which is secreted extracellularly to inhibit pathogen growth. These findings not only reveal the core biocontrol mechanism of DB02, namely the production and secretion of inhibitory VOCs targeting soil-borne pathogens, but also underscore its potential value for sustainable agriculture and integrated disease management. As an effective biocontrol agent, DB02 is a promising alternative to chemical pesticides and may contribute to improved crop quality.