Antifungal activity of methanol extracts of Leonotis nepetifolia L. and Ocimum gratissimum L. against ascochyta blight (Phoma exigua) on French bean

Abstract

The potential of crude plant extracts as antimicrobial agents has been demonstrated both in animal and plant health. The objective of this study was to evaluate the fungicidal efficacy of methanol extracts of Leonotis nepetifolia L. and Ocimum gratissimum L. against Ascochyta blight {Phoma exigua) pathogen in vitro. Composite leaves and tender stems were shade-dried, ground to fine powder using electronic hammer mill and extracted using methanol. Two-fold serial dilution was performed to obtain final concentrations of 40.0, 20.0, 10.0, 5.0, 2.5, 1.25, 0.63, 0.31 and 0.16 mg mL-1 (w/v) used in the laboratory bioassays against the pathogen. The experiment was performed according to Kirby-Bauer disc diffusion method in a Completely Randomised Design (CRD) in triplicate. The extracts showed antifungal activity at all the concentrations tested with Minimum Inhibitory Concentration (MIC) of 1.25 mg mL-1 for both L. nepetifolia and O. gratissimum. There was plant extract concentration-dependent increase in the diameter of zones of inhibition. The average diameter of zones of inhibition for the two plant extracts at 40.0 mg mL-1 were comparable to the synthetic fungicide (Carbendazim) used as positive control. These results showed that the two plant extracts possess antifungal activity against the pathogen and can be used as natural alternatives to the synthetic fungicides in the management of Ascochyta blight on French bean.