الفهرس 
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Abstract
Onion(Allium cepa) is an economically important crop in Egypt. Among the biotic stresses responsible for its low production during last few years, Sclerotium cepivorum Berk, caused onion white rot disease has caused serious economical losses to crop. The aim of the present study was to isolate and identify bioactive compounds from secondary metabolites of S. cepivorum. Successive extraction was done from culture filtrates in a separating funnel with chloroform and ethyl acetate respectively till complete exhaustion. The fungus biomass(mycelium and sclerotia of S. cepivorum) was extracted with methanol, the antifungal effects of culture filtrates and organic solvent extracts from secondary metabolites of S. cepivorum were evaluated against the same fungus. The highest inhibition of the fungal growth was obtained by concentration of 50%from culture filtrates; it reduced the fungal growth by 51.9%. This treatment has also caused reduction in the number of sclerotia by 98.6%.No sclerotial germination was observed on the media amended with the culture filtrates at all concentrations tested (10, 25 and 50%). Among all organic solvent extracts, methanol extract was the strongest growth inhibitor against the fungal pathogen; it inhibited S. cepivorum growth by 82.2, 93.3, and 93.6% when used at concentrations of 400, 800, and 1200 ppm, respectively. At the same concentrations, ethyl acetate extract exhibited high inhibition to the fungal growth i.e., 85.56, 88.89, and 90.56%, respectively. All organic solvent extracts completely suppressed the formation and germination of sclerotia at all concentrations. In greenhouse experiments, ethyl acetate extract was the most effective in reducing the incidence and severity of onion white rot disease after 60, 75 and 90 days. The chemical composition of methanol, ethyl acetate and chloroform extracts of S. cepivorum was determined by gas chromatography-mass spectrometry (GC-MS) , GC–MS analysis of the methanol extract led to the identification of 28 different compounds, representing 89.07% of the total extract. In the ethyl acetate extract there were identified ten compounds, which represent 59.12% of the total extract, and in the chloroform extract of eleven compound was identified which represent 55.27%. Bioactive compounds was isolated from secondary metabolites of S. cepivorum and identified by The Electron Impact Mass Spectra (EI-MS) , Nuclear Magnetic Resonance 1H-NMR and 13C-NMR spectra, Distortionless Enhancement Polarization Transfer (DEPT), Correlation Spectroscopy (COSY), Heteronuclear Multiple Quantum Coherence (HMQC) and Heteronuclear Multiple Bond Connectivity (HMBC). These compound were, Hexadecanoic acid methyl ester and Viridicatin.