AbstractSclerotium cepivorum (the causal agent of white rot of onions) persists in soil in the form of sclerotia whose germination is triggered by volatile sulphides and thiols released into the soil from roots of the genus Allium. The effects of treatment of the soil with diallyl disulphide (DADS) on the germination of these sclerotia has been studied, because if the germination can be triggered in the absence of the host crop, control of the fungus could be achieved. Five isolates of the fungus responded similarly but marked effects of DADS concentration and of the time‐of‐year of application were discovered, poor results being obtained with summer applications of DADS. Promising results were obtained at other times of the year with DADS flooded on to land containing sclerotia. The effectiveness of DADS treatment was shown to be related to the rate at which the material disappeared from soil, which was itself dependent upon soil temperature.