Extracts from aerial parts of Prosopis ruscifolia, Bidens pilosa, Cercidium praecox and Phoradendron liga were assayed against toxigenic Aspergillus species. They were obtained by sequential extraction of the aerial parts with hexane (fHex), dichloromethane (fDCM), ethyl acetate (fEtOAc) and methanol (fMeOH). The fMeOH from P. ruscifolia showed the highest antifungal spectrum (MIC = 750–1500 µg mL−1; MID = 50–200 µg; DI = 1.7–3.0 mm). Indolizidine alkaloids (juliflorine and juliprosine) and tryptamine were identified with strong (MIC = 188 µg mL−1) and moderate antifungal activities (MIC = 750 µg mL−1), respectively, towards A. parasiticus and A. flavus. The fMeOH, the indolizidine alkaloids and tryptamine synergized the fungitoxic effect of potassium sorbate and propiconazole. They completely suppressed the biosynthesis of aflatoxins at concentrations of 47, 94 and 375 µg mL−1, respectively. Our results indicate that fMeOH and its identified alkaloids are promisory additives of commercial antifungals and are antiaflatoxigenic agents at concentrations below of those required for complete suppression of fungal growth. © 2019 Informa UK Limited, trading as Taylor & Francis Group.