Secondary Metabolites from Juniperus Procera with Activity against Selected Maize and Bean Pathogens

Abstract

Maize (Zea mays) and beans (Phaseolus vulgaris L.) are important food crops in Kenya however, these crops have registered a decline in yields which is partly due to attack by fungal and bacterial pathogens. These pathogens are currently managed by use of synthetic pesticides, whose long term and widespread use has led to negative impacts which has endangered the environment in general. Extracts from Juniperus procera have been used as traditional medicine in treatment of various human pathogens. Scientific research has also proved efficacy of its various secondary metabolites towards human pathogens but minimal research has been done on their activity towards phytopathogens. This study, therefore, aimed at isolating secondary metabolites from the bark and leaves of J. procera with activity against three important plant pathogens; Xanthomonas axonopodis pv. phaseoli and Pseudomonas savastanoi pv. phaseolicola whose host is common bean and a maize pathogen Fusarium graminearum. The leaf and bark samples were collected from Mount Elgon National Park Forest, dried under shade and ground into a fine powder. Solvent extraction method was used to extract secondary metabolites from the powdered plant material whereby the powder was first soaked in methanol, filtered and solvent evaporated to obtain methanol extract. The methanol extract was suspended in water and partitioned between hexane and ethyl acetate to obtain ethyl acetate, hexane and aqueous extracts. The ethyl acetate extracts were purified using various chromatographic techniques. The pure compounds were analysed using 1D and 2D NMR and their masses determined using mass spectrometry. A flavonoid was isolated from the bark; epicatechin (12) while a biflavonoid and a diterpenoid were isolated from the leaf; podocarpusflavone A (13) and juniperolide (14) respectively. Disc agar diffusion assay was used to test the extracts and pure compounds against the three phytopathogens. Crude extracts were more active against the bacterial pathogens than the semi-pure fractions. For example, methanol and aqueous crude extracts from leaf had highest inhibition zones of 18.0 and 17.3 mm respectively against P. savastanoi pv. phaseolicola. Compound 12 had the highest inhibition zone of 21 mm against P. savastanoi pv. phaseolicola. Both compound 13 and 14 registered inhibition zones of 8 mm against P. savastanoi pv. phaseolicola. The extracts and the pure compounds did not show activity towards F. graminearum. It can therefore, be concluded that this medicinal plant can be a source of compounds to be used in control of important plant bacterial pathogens.