Populations of winter barley were sown in autumn in large tubs of soil to half of which additional phosphate was supplied. Half the plants of each phosphate treatment were infected with powdery mildew (Erysiphe graminis D.C. f. sp. hordei Marchal) and all plants were transferred to the-field. Infection induced extensive leaf injury during mid-winter in plants at low phosphate, but injury was greatly reduced in the high soil phosphate treatment. The extent of winter injury had a marked effect on the increase in leaf area in spring, and the accumulation of plant d. wt was positively correlated with the percentage of total leaf area remaining undamaged at the end of winter. This, in turn, was strongly influenced by the interaction between powdery mildew and soil phosphate. High soil phosphate may act as a ‘buffer’ to the effects of infection, minimizing the combined effects of infection and abiotic stresses suffered by plants in winter.