Josiah O. Omolo

Larvicidal activity of extracellular secondary metabolites from a Stereum species Hill ex Pers. (JO5289) against the dengue fever mosquito, Aedes aegypti (Linn) (Diptera: Culicidae)

The main objective of this investigation was to find mosquito larvicidal secondary metabolites from a basidiomycete – Stereum species (JO5289) – against Aedes aegypti . The Stereum species (JO5289) was collected in July 2005 from undisturbed habitat in Londiani forest in Rift Valley province, Kenya. Extracellular crude extracts from Stereum species (JO5289) produced strong activity against A. aegypti larvae. Purification of the crude extracts targeting larvicidal activity using chromatography gave three active compounds namely; tyrosol, 3-methoxy-5-methyl-1,2-benzenediol and 2-hydroxy-4-(4-hydroxychroman- 7yl) but-3-enal. The chemical structures of the compounds were determined using the nuclear magnetic resonance (NMR) spectral data and comparison with literature values. When tested for larvicidal activity, the LC 50 for the three compounds were 26.7, 17.3 and 14.5 ppm, respectively, while LC 90 were 85.3, 83.5 and 82.9 ppm, respectively, after 24 h of exposure. These compounds have been produced from cultures of a Stereum species and reported to have mosquito larvicidal activity for the first time. Key words: Basidiomycete, Stereum species, Aedes aegypti , extracellular metabolites and larvicidal.

In vitro inhibition of tomato Fusarium wilt causative agent by zearalenone from a soil inhabiting fungus

To find naturally occurring compounds for the control of Fusarium wilt disease of tomato in the farming fields, 200 fungal strains were cultured in liquid media and screened for antifungal activity against Fusarium oxysporium f. sp. Lycosperci . The screening was carried out using in vitro antifungal assays and coincidentally a strain of Fusarium species was found to produce secondary metabolites, which were prepared as crude extracts that showed strong antifungal activity. The responsible compound was purified using column chromatography and the chemical structure of the purified compound was determined using nuclear magnetic resonance (NMR) spectroscopic techniques. A known compound, zearalenone, was found to be responsible for the antifungal activity, with a minimum inhibitory concentration (MIC) of 550 ± 10.5 ppm. This is a sufficient inhibition for F. oxysporium f. sp. Lycosperci , which is the causative agent for Fusarium wilt disease to tomato. Given the global abundance of zearalenone and its implication to human health, the results from this study suggest a sustainable manner of disposing of the mycotoxin and simultaneous benefit in control of the Fusarium wilt disease. It is recommended from this finding that it can be used to suppress fungal attack onto the roots of tomato plant to manage the Fusarium wilt disease. Keywords: Fusarium oxysporium f. sp. Lycosperci , tomato, submerged cultures, column chromatography, zearalenone