Chemical profiling and inhibitory effects of selected South African plants against phytopathogenic bacteria and fungi of tomato

Abstract

Soilborne pathogens are economically important, causing great losses in agricultural production globally. The high cost and toxicity of antibiotics, coupled with the development of drug-resistant bacteria, has awakened interest in finding alternative methods of plant-pathogen control. This study aimed to screen extracts of selected plants against phytopathogenic bacteria and fungi that infect tomatoes, and to profile their chemical constituents. The antimicrobial activity of acetone, water and dichloromethane: methanol (DCM/MeOH = 1:1) extracts from leaves of ten plants was examined against five phytopathogenic bacterial strains and one fungal strain using a serial microplate dilution method to determine the minimal inhibitory concentration (MIC) values. Gas chromatography-mass spectrometry (GC–MS) was used for profiling constituents in the acetone and DCM/MeOH extracts. The MIC values indicated weak antibacterial activity of all the water extracts against tested bacterial strains. Acetone and DCM/MeOH extracts of Leucosidea sericea and Searsia lancea had very good to outstanding antibacterial activity against most of the tested bacteria with MIC values ranging between 19.5 and 78 µg/mL. All extracts were not active against Fusarium spp. except for the acetone extract of Cotyledon orbiculata and the water extract of Leonotis leonurus which inhibited the growth of F. oxysporum with MIC = 39 and 97.5 µg/mL after 24 h, with further incubation resulting in MICs of 156 and 469 µg/mL respectively. The GC–MS analysis of the acetone and DCM/MeOH extracts indicated the major peaks of 9-octadecenamide, (Z)-, octadecanoic acid and dodecanamide which were present in almost all the extracts. The 9-octadecenamide was found to be the most highly concentrated compound in most extracts. Leucosidea sericea and S. lancea, therefore, contain bioactive compounds that may be used as broad-spectrum antimicrobials against phytopathogenic bacteria and fungi.