Activity of three South African plants on phytopathogenic bacteria and fungi of tomatoes and chemical profiling of the extracts

Soil-borne pathogens in the fungal and bacterial kingdom cause destruction of large fields of agricultural crops, with great economic loss worldwide. The aim of this study was to evaluate three South African plant extracts for activity against phytopathogenic bacteria, fungi and to profile their chemical constituents. A serial microplate dilution method was used to determine the minimum inhibitory concentration (MIC) of Leonotis leonurus, Clausena anisata and Lantana rugosa crude extracts and fractions prepared using solvent-solvent fractionation. Antibacterial activity was determined against Xanthomonas perforans, Xanthomonas vesicatoria, Ralstonia solanacearum, Ralstonia pseudosolanacearum and Clavibacter michiganensis subsp. michiganensis (Cmm) while antifungal activity was evaluated against Fusarium oxysporum f. sp. lycopersici. Gas chromatography-mass spectrometry (GC-MS) was used for profiling the phytochemicals from the acetone and dichloromethane/methanol extracts. L. leonurus and L. rugosa extracts had moderate to weak activity with MIC values ranging between 0.156 to 2.5 mg/mL. L. rugosa fractions were more active with MIC values ranging between 0.078 to 0.156 mg/mL against most phytopathogenic bacteria, followed by dichloromethane and ethyl acetate fractions of L. leonurus. All extracts and fractions were inactive against Fusarium spp. except the water extract of L. leonurus with MIC of 0.156 mg/mL. The selectivity index indicated that L. rugosa was unsafe. The GC-MS analysis of L. leonurus dichloromethane/methanol extract revealed a high quantity (32%) of 9,12-octadecadienoyl chloride, (Z,Z)-. This was followed by 9-octadecenamide, (Z)- with 20%, and this compound is known to have antibacterial activity. This study supports further investigation of L. leonurus for management of pest diseases of tomatoes.