Investigation of secondary metabolites from Withania somnifera and hagenia abyssinica for the control of maize and bean pathogens

Abstract

Maize (Zea mays L.) and the common bean (Phaseolus vulgaris L.) are the two most important staple foods not only in Kenya but also in Sub-Saharan Africa. Despite the importance of these two crops, there have been production constraints due to diseases caused by bacterial and fungal pathogens. The major diseases affecting maize production are Gray Leaf Spot (GLS) and maize ear rot which are caused by fungal pathogens, Cercospora zeae-maydis, Fusarium moniliforme and Fusarium graminearum. Fungal pathogens, Colletotrichum lindemuthianum and Pythium spp. are the major causes of anthracnose and root rot diseases in beans respectively. Common bacterial blight (CBB) caused by Xanthomonas axonopodis pv. phaseoli and halo blight (HB) caused by Pseudomonas savastanoi pv. phaseolicola are among the most important bean bacterial diseases in Africa. This research investigated the activity of methanolic extracts from Withania somnifera, Hagenia abyssinica and essential oil (EO) from the leaves of H. abyssinica against these pathogens. The essential oil was extracted by hydro-distillation in a modified Clevenger apparatus. Disk diffusion method was used to determine the antimicrobial activities. The methanolic extracts of both plants (100 mg/ml) inhibited the growth of both bacterial and fungal pathogens. Extracts from W. somnifera had an inhibition zone of 12 mm and H. abyssinica 15 mm against C. lindemuthianum, which was much higher than the reference standard (Nystatin) inhibition zone of 10.6 mm. The activity of the essential oil was equal to that of standard Chloramphenicol (30 mm) against the two bacterial pathogens. Further isolation of the methanolic extracts through column chromatography yielded three compounds which were identified through 1D and 2D nuclear magnetic resonance. Two steroidal lactones were isolated from W. somnifera; 5α, 17β-dihydroxy-6α, 7β-epoxy-1-oxo-witha-2, 24-dienolide (19) and 4, 5, 6, 15 tetrahydroxy-1-oxo-witha-7-enolide (20). A pentacyclic triterpene, 2, 3, 19-trihydroxyurs-12-en-28-oic acid (21) was isolated from H. abyssinica. The isolated compounds, 19 and 20 from W. somnifera did not show any antimicrobial activity. This implies that the activity observed in the extract was due to synergistic effect, or other active compounds in the extracts. The GC-MS analysis of EO from the leaves of H. abyssinica revealed the oil was majorly constituted of sesquiterpenes (53.4%) and monoterpenes (33.2% of the total oil concentrations). The major sesquiterpenes were; ɗ-gurjunene (13.6%), aromadendrene (11.3%) and palustrol (6.2%) while major monoterpenes were endobornyl acetate (19.7%), camphor (3.6%) and azulene (3.9%). These findings show that these plants can be used as lead source of compounds in the development of new, biodegradable, environmentally friendly and cheaper antimicrobials.