SummaryThe results of a prolonged investigation of the fungicidal properties of solutions containing sulphur in polysulphide form upon the hop powdery mildew (S. Humuli) have been classified and summarised. During the course of this work the fungicidal efficiency of various lime sulphurs, liver of sulphurs, and solutions containing sodium, potassium, ammonium, calcium and barium polysulphides have been compared and it has been shown that, in all cases, the effect upon the fungus, when growing under similar biological and external conditions and provided that the sprays are applied under standardised conditions, is determined solely by the content of polysulphide sulphur of the spray fluid. The estimation of the amount of polysulphide sulphur present provides, therefore, a measure of the efficiency of these materials against S. Humuli.