Extracts of Stevia rebaudiana against Fusarium oxysporum associated with tomato cultivation

Abstract

The excessive application of agrochemicals to control phytopathogenic fungi has caused the development of resistant strains, damage to the environment and harmful effects on human health. The use of plant extracts represents an alternative to such chemicals because they have been shown to be effective in the control of phytopathogens and to be environmentally friendly since their secondary metabolites are biodegradable quickly. This study evaluated the antifungal activity of extracts of different polarity obtained from Stevia rebaudiana B. leaves against Fusarium oxysporum, a fungus that causes great losses in tomato cultivation. Hexane extract at a concentration of 833 ppm inhibited in vitro mycelial growth by more than 50%. Column chromatographic separation of the hexane extract resulted in a fraction with greater inhibitory activity and spectroscopic analysis of it revealed austroinulin to be the main component. In vivo tests with F. oxysporum infected tomato plants showed that all plants treated with the extract were taller, and had greater aerial and root dry weights that were significantly greather than those of control plants inoculated with F. oxysporum to which no extracts was applied. In treatments without F. oxysporum inoculum, applying hexane extract resulted in greater plant height and aerial dry weight. This indicates that the extracts contain secondary metabolites, including austroinulin, that not only inhibit the growth of F. oxysporum but that could also be acting as biostimulants. The aqueous, ethanol and hexane extracts added to tomato plants inoculated with F. oxysporum had an antifungal effect and were not phytotoxic.