Combined effects of powdery mildew disease and soil water level on the water relations and growth of barley

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An examination was made of the effects of powdery mildew disease (Erysiphe graminis hordei) on the water relations and growth of barley plants maintained at three different soil water levels in a climate chamber. In uninfected controls the growth of vegetative organs and transpiration per plant were greatest, and leaf water and solute potentials were highest, in plants grown in wet soil; the same parameters were respectively smallest and lowest in plants grown in dry soil.Infection late in the development of plants (ears emerged) had no effect on growth, but caused a small increase in leaf senescence and water consumption. When infection occurred earlier in development, mildew and lack of soil water had additive, harmful effects on plant growth, although the inhibition of root and shoot growth caused by mildew was proportionately less when plants were growing in dry soil than when they were growing in wetter soils, a difference that could not be attributed to differences in fungal development.Mildew did not inhibit growth by deleteriously affecting the water relations of the plant, for infection neither induced water stress in well-watered plants nor exacerbated the stress already existing in plants in dry soil. Infection increased the shoot to root ratio in plants in wetter soils but had no effect on the ratio when the plants were in dry soil, where such an increase would have been most damaging. Water consumption per plant was reduced by infection because the transpiring area of the plant was reduced and, also, because stomatal opening in the light was inhibited in infected tissues.At all soil water levels infection reduced 1000 grain weight, number of grains per ear and incidence of survival to maturity of the two main shoots. Yield of infected plants, relative to that of uninfected controls, was reduced least when plants were grown in dry soil, although this may have been due partly to the restricted development of mildew on the upper leaves of plants in dry soil.