The effects of powdery mildew (Erysiphe graminis f. sp. tritici ) on the carbohydrate metabolism of wheat leaves was studied during infection. During infection the triazole fungicide hexaconazole (trade name ANVIL, Zeneca Agrochemicals) was applied to leaves to determine if removal of the fungal sink reversed any pathogen-induced alterations in host physiology. Five days after inoculation there was a large accumulation of sucrose, glucose, fructose and hexose-phosphates and an increased partitioning of current photosynthate into starch rather than sucrose in both mildewed leaves and those treated with fungicide 2 and 3 days after inoculation. Within 48 h of inoculation, acid invertase activity bad doubled and this increase was maintained until 7 days after inoculation. Application of hexaconazole to a mildewed leaf caused a transient decline in invertase activity but over subsequent days activity increased again until it resembled that observed in a mildewed leaf. Hexaconazole severely inhibited fungal development and hence the size of the fungal sink. This suggests that the enhanced invertase activity was triggered by a signal from the fungus but was not directly related to the size of the fungal sink. These results suggest that (i) the stimulation of invertase activity in mildewed leaves altered the source-sink relationship of the leaf leading to an accumulation of soluble carbohydrates and a decrease in sucrose synthesis and (ii) that application of hexaconazole successfully prevented development of the fungus but that the metabolic alterations which occurred following the initial infection were not reversible within the time scale studied.