Efficacy of Leonotis nepetifolia l. and Ocimum gratissimum l. extracts in controlling ascochyta blight (Phoma exigua) of french bean (Phaseolus vulgaris l.)

Abstract

French bean (Phaseolus vulgaris L.) is an important export vegetable crop that faces serious production challenge due to Ascochyta blight (Phoma exigua). The objectives of this study were to determine the; (i) antifungal activities of methanol extract of Leonotis nepetifolia and Ocimum gratissimum against Phoma exigua in vitro, (ii) fungicidal effects of extracts of the two test plants on the incidence and severity of Ascochyta blight, and yield of French bean. Methanol extracts of composite leaves and tender stems of each test plant were diluted 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) for use in antifungal tests. The pathogen was isolated from infected French bean pods and used for inoculation in greenhouse and field experiments. Laboratory bioassays were conducted using Kirby-Bauer disc diffusion method in a completely randomised design in triplicate. The diameters of zones of inhibition and the minimum inhibitory concentrations of the methanol extracts against the pathogen were determined. The in vivo experiments were conducted at Egerton University and Kenya Agricultural and Livestock Research Organization – Njoro fields. The experiments were laid in a 2 × 6 factorial arrangement of randomized complete block design with three replicates. Methanol extracts of O. gratissimum and L. nepetifolia at 20.0, 10.0, 5.0, 2.5 and 1.25 mg ml-1 were sprayed per plot at 14 days after emergence (DAE). The in vivo data were collected on disease incidence and severity, yield; and percent disease index (PDI) and disease severity index (DSI) determined per plot. The data were subjected to analysis of variance using Statistical Analysis System software and treatment means separated using Tukey’s HSD test. Correlation analysis between the botanical rates, disease incidence and severity, and yield per plot was conducted to determine the effect of the test plants on yield of French bean. 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 were concentration-dependent antifungal activities of extracts of the test plants with average inhibition zones (at 40.0 mg ml-1). Ocimum gratissimum and L. nepetifolia at 5.0 mg ml-1 resulted in 4.2 and 12.5% reduction of disease incidence, respectively. Ocimum gratissimum at 10.0 mg ml-1 and L. nepetifolia at 20 mg ml-1 caused 13.5 and 13.1% reduction in Ascochyta blight severity in the field. Use of methanol extracts of the two test plants resulted in a significant increase (between 42.6 – 91.4 and 49.8 – 95.7 % for Ocimum gratissimum and Leonotis nepetifolia, respectively) in yield of French bean per plot. These findings provide insight on the potential of the two plant extracts to control Ascochyta blight and to promote low risks, cheap bio-pesticides for the smallholder farmers.