Impact of Oxalic Acid Consumption and pH on the In Vitro Biological Control of Oxalogenic Phytopathogen Sclerotinia sclerotiorum

The phytopathogenic fungus <i>Sclerotinia sclerotiorum</i> has a wide host range and causes significant economic losses in crops worldwide. This pathogen uses oxalic acid as a virulence factor; for this reason, the degradation of this organic acid by oxalotrophic bacteria has been proposed as a biological control approach. However, previous studies on the potential role of oxalotrophy in biocontrol did not investigate the differential effect of oxalic acid consumption and the subsequent pH alkalinisation on fungal growth. In this study, confrontation experiments on different media using a wild-type (WT) strain of <i>S. sclerotiorum</i> and an oxalate-deficient mutant (strain Δ<i>oah</i>) with the soil oxalotrophic bacteria <i>Cupriavidus necator</i> and <i>Cupriavidus oxalaticus</i> showed the combined effect of media composition on oxalic acid production, pH, and fungal growth control. Oxalotrophic bacteria were able to control <i>S. sclerotiorum</i> only in the medium in which oxalic acid was produced. However, the deficient Δ<i>oah</i> mutant was also controlled, indicating that the consumption of oxalic acid is not the sole mechanism of biocontrol. WT <i>S. sclerotiorum</i> acidified the medium when inoculated alone, while for both fungi, the pH of the medium changed from neutral to alkaline in the presence of bacteria. Therefore, medium alkalinisation independent of oxalotrophy contributes to fungal growth control.