<i>Tecoma stans</i> (Bignoniaceae), leaf extracts, fractions and isolated compound have promising activity against fungal phytopathogens

Abstract

Plant pathogenic fungi are a major cause of reduced plant production and post-harvest losses of plant produce. The control of these fungi by some synthetic fungicides is complicated by human and environmental toxicity, the development of resistance by some fungi and high costs, thus prompting the investigation of other means of fungal control. Plant secondary metabolites have a good potential as antifungal agents. The aim of this study is to investigate the potential use of Tecoma stans as a plant-derived fungicide by determining the antifungal activity of extracts, isolating the bioactive compound and testing the toxicity of both the extract and the isolated compound. In bioassay-guided fractionation of the leaves of the Tecoma stans dichloromethane (DCM) extract contained one major compound that was isolated and characterised as oleanolic acid. The DCM extract and oleanolic acid were active against 10 tested plant fungal pathogens (Aspergillus niger, Aspergillus parasiticus, Collectotrichum gloeosporoides, Fusarium oxysporium, Penicillum expansum, Penicillum janthinellum, Pythium ultimum, Phytophthora nicotiana, Trichoderma harzianum and Rhizoctonia solani) with an average minimal inhibitory concentration of 130 μg/mL. The DCM extract and oleanolic acid were toxic to Vero cells with an LC₅₀ of 0.413 mg/mL and 0.129 mg/mL respectively, when compared with berberine, a toxic compound with LC₅₀ of 15.48 µg/mL. Oleanolic acid was more toxic than the crude extract, supporting the potential use of plant extracts for controlling plant fungal pathogens. The selectivity indices of 20 with several fungi indicated that extracts could possibly be used under controlled conditions against infections of certain fungal pathogens, even on edible plants. The large quantities available of this invasive plant species could lead to a commercially useful product in controlling plant fungal pathogens.

Research correlation: This article is the translated version, made available to provide access to a larger readership, of which the original English article is available here: https://doi. org/10.4102/satnt.v36i1.1496