Hydroponic experiments were conducted to compare the effects of 'Pythium irregulare' and root pruning on wheat ('Triticum aestivum' cv. Janz) transpiration, water-use efficiency (WUE) and plant growth in the presence and absence of polyethylene glycol-induced drought (PEG). 'Pythium', PEG and root pruning reduced transpiration to a similar extent, but the mechanism that affects transpiration differed between the treatments. Reduced hydraulic conductivity of roots caused by disease in the 'Pythium' treatment and reduced size of the root system in the root pruning treatment were responsible for decreased transpiration while reduction of stomatal conductance was the main cause for reduced transpiration in the PEG treatment. 'Pythium' reduced shoot dry weight and increased root/shoot ratio but had no effect on whole-plant or instantaneous WUE. There was a small additive effect of 'Pythium' on whole-plant transpiration of plants exposed to PEG-induced drought, but there was no evidence of an interaction between 'Pythium' and PEG-induced drought on WUE or growth. This suggests that moderate root damage by pathogens is likely to have only a modest effect on the water relations of wheat plants.