PROSPECTS FOR USE OF ALIEN INVASIVE WEED EXTRACTS AGAINST FUNGAL PHYTOPATHOGENS

Abstract

Plant pathogens, in particular plant fungal pathogens, cause a major threat to food security. There are about 5 000 fungal species that destroy economically valuable crops. Plant pathogenic fungi attack most crops in the field and also post-harvest, therefore decreasing their production and their shelf life. The most important method of protecting plants against fungal attack is the use of synthetic fungicides, however the development of resistance towards the use of these fungicides is of great concern. This has spurred on research for search for safe, efficacious and environmentally friendly fungicides. Invasive weeds over time have shown resistance towards fungal pathogens. The possibility that these weeds may contain active principles to resist fungal attack should be considered. Previous work investigating Melianthus comosus indicated excellent antifungal activity, but problems in cultivating the species prompted our investigation of readily available invasive weeds. After seven invasive species (Chromolaena odorata, Ipomoea alba, Tecoma stans, Passiflora suberosa, Passiflora subpeltata, Aristolochia sp., Solanum seaforthianum) were screened against 10 plant fungal pathogens, Rhizoctonia solani, Fusarium oxysporium, Penicillum janthinellum, P. expansum, Aspergillus parasiticus, Colletotrichum gloeosoariiodes, Trichoderma harzianum, Pythium ultimum and Phytophthora nicotiana. Acetone extracts of Tecoma stans had promising antifungal activity with an average minimal inhibitory concentration (MIC) value against all the fungi as low as 0.55 mg/ml. Bioassay-guided fractionation of the leaves of the Tecoma stans dichloromethane (DCM) extract resulted in the isolation of one major compound, oleanolic acid. This compound had good antifungal activity with an average MIC value as low as 0.13 mg/ml against the 10 plant pathogenic fungi and clear bands against all tested fungi on bioautograms, indicating fungal growth inhibition. Cytotoxicity of the isolated compound was investigated using the Vero monkey cell line and gave an LC50 value of 0.1296 mg/ml. This study shows that weeds may be a source of active antifungal extracts or compounds active against phytopathogenic fungi. Acknowledgements: University of Pretoria (UP), National Research Foundation (NRF).