In vitro phytotoxicity of culture filtrates of Fusarium oxysporum f. sp. vanillae in Vanilla planifolia Jacks

The fungus Fusarium oxysporum f. sp. vanillae is the cause of stem and root rot in Vanilla planifolia, the most devastating disease of this crop. As part of a biotechnological breeding program to obtain vanilla genotypes resistant to this pathogen, we evaluate here the phytotoxic effects of culture filtrates of F. oxysporum f. sp. vanillae during the multiplication and rooting of plantlets of V. planifolia. In this research, the in vitro phytotoxicity of culture filtrates of three strains of the fungus together with their capacity to synthesize fusaric acid was evaluated. The median lethal dose (LD50) of the strains was determined by adding various concentrations of the fungal culture filtrates by means of in vitro cultivation of vanilla and calculating the percentage survival at the multiplication and rooting stages. The drastic decrease in the survival percentages of explants, denotes that only the H3 strain has phytotoxic effects in the two phases (multiplication and rooting) in vitro culture of V. planifolia. The strain H3 produced the highest level of fusaric acid (FA). During the multiplication phase and rooting, the FA level was inversely correlated with the survival of shoots. This study contributes to future programs of in vitro selection of vanilla genotypes resistant to this pathogen.