Evaluation of Yield, Chemical Composition and Antimicrobial Activity of Essential Oils of Tagetes Minuta L. (Asteraceae) Against Selected Phytopathogens

Abstract

Phytopathogenic fungi and bacteria cause enormous crop yield losses worldwide. Over the years, management of plant pathogens has primarily relied on the use of synthetic chemical antimicrobials and development of disease resistant varieties. Exploration for biologically active compounds from plants with an aim of discovery and development of novel and eco-friendly biopesticides to combat current and emerging phytopathogens has received increased interest in the recent past. This study aimed at extraction of Tagetes minuta essential oils (EOs), evaluation of the antimicrobial activity of the essential oils against selected phytopathogenic fungi and bacteria and characterization of chemical composition of the essential oils. The antimicrobial activity of the essential oils was evaluated against five plant pathogenic fungi: Fusarium oxysporum, Fusarium solani, Aspergillus flavus, Aspergillus parasiticus and Aspergillus niger; as well as three plant pathogenic bacteria: Pseudomonas savastanoi pv. phaseolicola, Xanthomonas axonopodis pv. phaseoli and Xanthomonas axonopodis pv. manihotis. Aerial parts (leaves, flowers and stems) of Tagetes minuta plant materials were sampled from four sites within Maseno area, Kisumu County, Kenya. Extraction of EOs was carried out using the steam distillation method in a modified Clevenger-type apparatus. The antimicrobial activity of the oils was assessed by disc diffusion method while gas chromatography - mass spectrometry (GC/MS) was used for characterization of chemical components of EOs. A mean oil yield of 0.0594% w/w was obtained while GC/MS analysis identified 20 compounds corresponding to 96% of the total essential oil. The EOs mainly contained a mixture of monoterpenes (70%) and sesquiterpenes (30%). The most abundant monoterpenes were (E)-Tagetone (11.8%), dihydrotagetone (10.7%), Allo-Ocimene (8.8%), (Z)-β-Ocimene (7.0%) and Limonene (5.3%). Sesquiterpenes concentrations were in the range of 3.4 to 3.5%. Among the identified compounds were Elixene and Silphiperfol-6-ene, two sesquiterpenes being reported for the first time in essential oils of T. minuta. The EOs exhibited potent antimicrobial activities against all the studied pathogens. In the case of fungi, the highest activity of the oils was observed in F.oxysporum and A. niger where mean inhibition zones of 28.67mm were recorded after five days of incubation. The EOs minimum inhibitory concentrations and minimum fungicidal concentrations were in the ranges of 24 - 95mg/ml and 24 - 190mg/ml, respectively. Pseudomonas savastanoi pv. phaseolicola was the most susceptible bacteria to the EOs, with a mean inhibition zone diameters of 41.83 and 44.83mm after 24 and 48 hours, respectively. The EOs minimum inhibitory concentrations and minimum bactericidal concentrations were in the ranges of 24 - 48mg/ml and 95 - 190 mg/ml, respectively. These findings provide the scientific basis for the use of T. minuta essential oils in the management of economically important plant pathogens. This study thus lays down significant groundwork for a more comprehensive study on the practical feasibility of using Tagetes minuta EOs as biopesticide. It is hoped that such studies would potentially promote the development of novel, affordable and eco-friendly biopesticides for management of economically important phytopathogens.