Isolation of antifungal secondary metabolites from Phytolloca dodecandra, Basella alba and Lippia javanica against maize fungal pathogens

Abstract

Maize is an important cereal crop and a staple food in most parts of Africa. Food insecurity is one of the major challenges in Sub-Saharan Africa. The food situation is worsened by fungal infestation of maize crops in the fields. The most prevalent fungi on maize crops are Fusarium moniliforme and Fusarium graminearum that cause red ear rot, stalk rot and Grey leaf spot (GLS) respectively. The development of antifungal resistance and side effects associated with synthetic pesticides has triggered intense research efforts towards natural antifungal agents such as essential oils and the non-volatiles because of their reported efficacy and safety. In an effort to search for new antifungal agents, selected plants; Phytolloca dodecandra, Basella alba and Lippia javanica were screened for bioactivity against maize pathogens; F. moniliforme and F. graminearum. The research involved isolation of secondary metabolites from the selected plants and extraction of essential oil from L. javanica. The essential oil was extracted by hydro-distillation and its chemical compositions determined by Gas Chromatography-Mass Spectrometry. Monoterpenes which accounted for 43.48% dominated the oil while the Sesquiterpenes constituted 8.70% respectively. The paper-disc diffusion-inhibition test method was used to screen for antifungal activity of both volatile and non-volatile secondary metabolites. Potato Dextrose Agar (PDA) was used as the culturing media for the fungi. The oil showed the highest activity of 25.00mm at 0.87 g/mL as compared to that of the Nystatin16.00mm, which was the positive control. The antifungal activity of the oil was observed to be reducing with time. On the other hand, the non-volatile secondary metabolites showed no activity against the selected phytopathogenic fungi. Bioassay guide column fractionation was done on the ethyl acetate extracts for the plants P. dodecandra and B. alba. From the two plants, one compound was isolated from each. Compound 16 was isolated from P. dodecandra and compound 17 isolated from B. alba. The compounds did not show any observable activity against the selected fungal pathogens.