Search for antifungal compounds from extracts of basidiomycetes against phytopathogen Fusarium oxysporium f. sp. lycopersici

Abstract

Tomato is one of the most widely cultivated horticutural crops but the production is affected by the pest problems like Fusarial wilt. Fusarium oxysporium f. sp. Lycopersicit is a destructive disease of the tomato and one of the main causes of the crop decline worldwide. The disease is currently managed using synthetic fungicides but there is a growing concern about the traces of the pesticide residues on the product and resistance to the current fungicides. Fungicides are applied at various stages of the tomato to protect the plant from fungal diseases but there is great concern due to environmental pollution and health problems. Therefore, alternative fungicides from naturally occurring compounds that are biodegradable offer a great potential for the control of crop fungal pathogens. Fungi are known to be prolific producers of secondary metabolites that have a wide range of beneficial biological activities, which can be researched on for crop protection. In this work, the fungi investigated were collected from different ecological niches in Kenya and kept on agar slants in the IBRL at Egerton University. For each strain collected, crude extracts were prepared from sterile submerged liquid nutrient media where growth conditions were manipulated to trigger production of secondary metabolites. In activity guided screening, Fusarium oxysporium isolate from a tomato plant, was used as the target organism in agar diffusion assay. From the ongoing screening of 400 crude extracts about 5% showed significant and interesting activity. One species, which showed significant and reproducible activity, was purified for natural antifungal compounds from its submerged cultures. The selected strain JO5125 was cultivated in constituted liquid nutrient media and crude extracts prepared from both the broth and mycelia at the end of the fermentation. The crude extracts were fractionated using both silica gel and reverse phase liquid chromatography to eventually lead to purification of the responsible compounds. The structure elucidation was determined using 1- and 2-D NMR spectroscopic techniques. The purified compounds were zearalenone and 3,4-dimethoxyphenol, which gave a minimum inhibitory concentration against the pathogen of about 550 and 500ppm, respectively. They are naturally occurring compounds which qualify as control agents of F. oxysporium with minimum impact in environment.