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.

The hypothesis that root rot caused by Pythium irregulare reduces the water use efficiency of wheat was tested in a system which simulated field conditions with late season water stress. Inoculation with Pythium significantly reduced transpiration during vegetative growth, so that plants entered post-anthesis drought with more available water. Although weekly transpiration rates were higher in inoculated plants than controls during the later stages of drought, infected plants were unable to make use of all of the extra water. There were no significant effects of inoculation on shoot biomass or grain yield, while total transpiration was reduced by 14%. Infected plants therefore had significantly higher integrated water use efficiency (grain yield relative to transpiration) than controls. Infected plants were significantly more stressed than controls during the drought, despite higher soil moisture, and showed reduced ability to use stomatal conductance to regulate leaf water potential. Pythium infection caused adverse changes to plant water use and water relations, but these did not translate into reductions in growth or yield. This, and the unexpected increase in water use efficiency, highlights the need to consider interactions with other environmental stresses when making assumptions about the effects of root diseases on crop productivity.

A pot experiment was conducted to investigate the effects of fungal root diseases ('Pythium' and 'Rhizoctonia') and drought at either tillering or anthesis on the water-use efficiency (WUE), water relations, yield components and percentage of root lesion of two Australian bread-wheat cultivars (Mulgara and Janz). There were no significant differences between two pathogens. WUE did not differ between well-watered plants and those droughted at tillering but it was significantly reduced by drought at anthesis. Mulgara had slightly higher WUE than Janz. Drought at both growth stages significantly altered water relations for both cultivars. Uninfected plants of Janz droughted at tillering had higher total water potential (Ψ) and osmotic potential (π) than diseased plants. However, osmotic potential of droughted controls was lower than diseased plants at anthesis. The number of heads, grain weight and grain number were significantly higher for well-watered than droughted plants and higher for Mulgara than Janz. The controls (38%) had significantly less lesioned roots than 'Pythium' (53%) and 'Rhizoctonia' (50%) and root lesion percentage in Janz was significantly higher than Mulgara except in the 'Rhizoctonia' treatment. In conclusion, the pathogens affected water use during tillering but not at later stages when roots developed beyond the inoculation point.