Allium white rot caused by soil-borne fungus Sclerotium cepivorum is a major problem for the onion-growing industry in many countries. The sclerotia of the fungus are disseminated by high winds, flooding, irrigation and infected plant material. Disease severity depends on sclerotia levels in the soil at onion planting period. Symptoms include foliage yellowing, dieback and wilting, in addition to root rotting. Sclerotium consists of firm aggregates of vegetative hyphae which serve as a means of the fungus survival. S. cepivorum microconidia are considered as sterile structures, however sclerotia are the only known dissemination unit. The disease disseminates clonally with relatively low genetic diversity in yield populations. The process of fungal penetration, sclerotia ultrastructure, the methods of fungal controlling, and the efforts to select and develop onion germplasm to resist Allium white rot are reviewed. The genetic diversity at epidemiological, physiological, biochemical and molecular levels are discussed with the aim of understanding the biology, epidemiology and genetics of S. cepivorum; also the association between genetic variability and virulence, durability of sclerotia, the germination ability of sclerotia, reaction to fungicides and ability to resist a biological control agent are discussed.