Several plants use secondary metabolites against insects and pathogens attack. Among such metabolites known glycoalkaloids are the most studied. Plants from Solanaceae Family are the most abundant on those alkaloids. Despite alfa-tomatine is a known glycoalkaloid, its specific action against Moniliophthora perniciosa, fungi responsible for witches broom disease in cocoa plantations was obtained in this work. Alfa-tomatine was infusion-extracted from Solanum lycopersicum leaves, and its action against the fungi growth was shown on the bioassays. In these tests, M. perniciosa growth was totally inhibited but other fungus tested, the inhibition was not totally, or did not present effect on growth, showing in this way, the specificity of alfa-tomatine to M. perniciosa. The advantages of work with a natural molecule for the fungi control is the stability of the molecule, its resistance to high temperature and pressure and for its use, the simple method of extraction used. The confirmation of the alfa-tomatine molecule was done by High Performance Liquid Chromatography and Mass Spectrometry in comparison to the Sigma standard.