A plant antifungal product from Melianthus comosus (Melianthaceae) leaf extracts

Melianthus comosus Vahl growing widely in southern Africa is used traditionally to treat bacterial infections. A company was interested in developing an antibacterial product for the veterinary market. The antibacterial activity of extracts was not high enough to pursue any further. It had excellent antifungal activity against animal pathogens, but the toxicity would have complicated the development of a product.

Extracts had an excellent activity against 10 plant fungal pathogens investigated (Rhizoctonia solani, Fusarium oxysporum, Penicillium janthinelum, Penicillium expansum, Colletotrichum glocosponicales, Trichoderma harzianum, Pythium ultimum, Phytophthora nicotiana, Aspergillus niger, and Aspergillus parasiticus). The extract contained one major antifungal compound and this compound was isolated and characterized as 3-hydroxy-12-oleanen-30-oic acid.

By selective extraction and solvent fractionation an extract with an average MIC of 0.066mg/mL against all ten fungal pathogens was obtained. Ignoring MIC values of 0.16mg/mL against Penicillium expansum and Aspergillus niger, the average MIC for the other fungi was 0.04mg/mL. The acetone extract did not lose activity at room temperature for a month. The dried extract was slightly soluble in water and ethanol, reasonably soluble in ethyl acetate and highly soluble in acetone. The potentised extract had a higher antifungal activity than six commercially used fungicides against some important plant fungal pathogens. In a limited field trial it gave a much better result than a commercial fungicide even though it was used at a quarter of the dose of the commercial fungicide. The results have been patented and a product is under development.

Acknowledgements: Healthtechlaboratories and THRIP provided funding.