Phytopathogenic Fungal Growth Inhibition by Actinomycetes Isolated from the Rhizosphere of Cymbopogon citratus (Lemongrass)

The search for natural antimicrobial substances to mitigate the deleterious effects of pathogens is on the increase due to the spread of resistance. In the agricultural sector, the current focus is on natural, eco-friendly, and sustainable approaches to improve crop yield. In this study, sixty actinomycetes isolates belonging to 3 genera; Streptomyces, Nocardia, and Micromonospora, obtained from 30 lemon grass rhizosphere samples, were screened for antifungal activity using three common fungi phytopathogens; Fusarium solani, Aspergillus flavus and Alternaria alternata. Preliminary screening involving the in vitro dual culture assay and the agar well diffusion techniques were employed to investigate the inhibitory activities of actinomycetes isolates and the ethyl acetate extract of their metabolites against the phytopathogens. Results obtained revealed that mycelial extensions in the fungi were significantly inhibited, though, to varying extents. Among the 60 actinomycetes isolates screened, 19 (31.37%) demonstrated significant inhibitory activity against at least one of the three test fungi. Out of the 19 isolates, 5(26.3%) suppressed mycelial growth in all three test fungi with percent mycelial inhibitions ranging from 51.63 – 69.29 %. Also, the crude extracts of the metabolites of these five actinomycetes isolates produced mycelial inhibitions ranging from 75.45 - 89.31 %. Analysis of the crude extracts using gas chromatography, revealed the production of five antifungal compounds which include: Benzylaldehyde,2-nitro-diaminomethylidene hydrazine, [1,2,4] triazolo [1,5] pyrimidine -6-carboxylic acid, 1,2, - benzenediol,3,5-bis(1,1-dimethylethyl), Benzo[h] quinolone,2,4-dimethyl-, and 2,4,6-cycloheptatrien-1-one, 3,5, bis-trimethylsilyl-. Therefore, the antifungal capabilities of these actinomycetes isolates make them efficient, natural, and ecologically friendly alternatives in the control of fungal phytopathogens.