Seed priming with co-aggregated Azospirillum and Methylobacterium cells reduce the bacterial fruit blotch incidence in watermelon

Abstract Microbial inoculants are increasingly vital in the plant growth-promotion and disease management of important agricultural crops. The possibility of co-aggregated Azospirillum sp. (CW903) and Methylobacterium sp. (CBMB110) as a biocontrol approach to combat watermelon blotch disease was investigated in this study. In survivability assays, co-aggregated CW903 outperformed CBMB110 in the watermelon soil. The co-inoculation using CBMB110 and CW903 in the co-aggregated form drastically reduced the number of Acidovorax citrulli on watermelon leaves and lowered the lesion areas by 42%. Various physiological and biochemical parameters such as phenol concentration, ethylene emission, and trans-cinnamic acid, were significantly decreased by the co-inoculation. Significant decreases were also observed in electrolyte leakage, H₂O₂ concentration, β-1,3-glucanase activity, phenylalanine ammonia-lyase (PAL) activity, peroxidase (POD), and polyphenol oxidase (PPO) activity. Co-aggregated cells enhanced disease suppression efficiency, and significantly reduced the severity of seedling blight by 21% and leaf spot by 35%.