Bioassay Guided Fractionation of Withania somnifera for the Management of Ascochyta rabiei

Withania somnifera (L.) Dunal., a medicinal plant of family Solanaceae, was used to examine its antifungal activity against Ascochyta rabiei, the causal organism of highly damaging blight disease of chickpea (Cicer ariatinum L.). In laboratory screening bioassays, different concentrations (0.2, 0.4, 0.6, 0.8, 1.0 g 100 mL(-1)) of methanolic extracts of stem, root, leaf and fruit of W. somnifera were evaluated for their antifungal activity against the targeted fungal pathogen. The methanolic extracts of fruit and leaf were found highly effective against A. rabiei causing up to 41% and 43% suppression in fungal biomass, respectively. The methanolic extract of fruit was further subjected to fractionation by using n-hexane, chloroform, ethyl acetate and n-butanol in order of increasing polarity. Different concentrations (3.125, 6.25, ..., 200 mg mL(-1)) of these fractions were used against the test pathogen. Ethyl acetate fraction was found highly effective against A. rabiei exhibiting complete inhibition of fungal growth. Two compounds namely A and B were isolated from this fraction through thin layer chromatography (TLC). These compounds were then appraised to find minimum inhibitory concentration (MIC) against the test fungal pathogen. Compound B was found more effective than A. The MIC values of compounds B and A were 62.5 mu g mL (-1) and 1000 mu g mL(-1), respectively. It was concluded that antifungal compounds B could be used for the management of A. rabiei. (C) 2012 Friends Science Publishers