Antifungal lignans from the creosotebush (Larrea tridentata)

Aspergillus flavus and Aspergillus parasiticus are preharvest pathogens of several important food crops, including maize. Both fungi produce aflatoxins, which are a potent hepatotoxin and carcinogen. The growth of A. flavus and A. parasiticus and subsequent aflatoxins contamination are sometime unavoidable, when the grains are grown under stressful conditions such as drought. The ideal control approach is prevention of fungus growth in the field. The objectives of this study were: (a) to evaluate the antifungal properties of Larrea tridentata extract against A. flavus and A. parasiticus; (b) to isolate and to identify the antifungal constituents from L. tridentata. Assay-guided chromatography of the Larrea tridentata extracts resulted in the isolation of two naturally occurring lignans, methyl-nordihydroguaiaretic acid (methyl-NDGA) and nordihydroguaiaretic acid (NDGA). Their chemical structures were identified by comparison of their physical and spectroscopic data (mp, 1H NMR, 13C NMR, and MS). The antifungal activity of the lignans was evaluated by radial growth inhibition assay against A. flavus and A. parasiticus. Methyl-NDGA was very effective in inhibiting mycelial growth of both fungi at 300 μg/ml, whereas 500 μg/ml of NDGA was necessary to completely inhibit the growth of the fungi. These compounds may have a biopesticidal potential as control agents for the aflatoxin fungi.