Nine isolates of Sclerotium cepivorum obtained from different localities of Upper Egypt were able to infect Giza 6 onion cultivar and produced typical symptoms of white rot. The pathogenic potentialities of the tested isolates ranged from moderate to severe. Trichoderma viride and T. harzianum were capable to antagonize growth of the tested S. cepviroum isolates in vitro. They showed different inhibitory effect against growth of the pathogen isolates on PDA medium ranged from moderate to severe. Under greenhouse conditions treated infested soil with the pathogen isolates No. 5 and 7 with both antagonists at transplanting time, significantly reduced percentages of infection and white rot severity on Giza 6 onion cultivar and numbers of the pathogen sclerotia in soil. The least disease severity % and the lowest numbers of sclerotia in soil were achieved by T. viride. Treated Giza 6 onion plants with combined treatments of the biocide (BioHealth) and the plant growth promoters (Algarin and Inicium) caused significant reduction in incidence of onion white rot in greenhouse and in field plots (naturally or artificially infested with pathogen isolates). Soil treatment with single or combined treatments of T. viride or T. harzianum and the tested organic sulphides one week before transplanting date caused significant reduction in both disease incidence and numbers of the pathogen sclerotia in infested soil with the pathogen isolates. The lowest disease severity % and numbers of the pathogen sclerotia in soil were achieved by combined treatments (organic sulphides + antagonists) compared with single treatment with each of them alone.