A fertilizer experiment on potatoes in which mechanical damage was followed by infection with grey mould (Botrytis cinera Pers. ex Fr.) provided the opportunity to make some observations on the incidence of the disease upon plants having different levels of mineral nutrition. It is suggested that the application of potash increases the resistance to infection by decreasing the host susceptibility to damage. The response to nitrogen may be governed by the pathogen's requirement for both nitrogen and carbohydrate.