Oxalic acid produced by Aspergillus niger Y-1 is effective for suppression of bacterial fruit blotch of watermelon seedlings

Bacterial fruit blotch (BFB) of watermelon caused by Acidovorax citrulli (Ac) is a seedborne disease. Seed treatment with bacterial disinfectants is considered as an important measure for suppression of Ac infection. This study was performed to detect the antibacterial activity of the cultural filtrate (CF) of the saprophytic fungus Aspergillus niger Y-1 against Ac. The six-day-old CF, citric acid (CA, 4 mmol/L) and oxalic acid (OA, 60 mmol/L) were determined for in vitro antibacterial activity against Ac. CF and OA were determined as seed disinfectants for suppression of seedborne infection by Ac. Results showed that production of CA and OA by A. niger in potato dextrose broth was consistently detected by HPLC. The CF, CA and OA inhibited growth of Ac and their inhibitory effect disappeared when the pH values of the three solutions was adjusted to 7.0. OA was more effective than CA in suppression of Ac. The potting experiment showed that both CF and OA applied on watermelon seeds effectively suppressed BFB incidence on seedlings. The efficacy was comparable to the seed treatment with HCl (1%, v/v). This study suggests that the CF of A. niger and OA can be used as seed disinfectants for elimination of seedborne Ac.