SUMMARYPot‐grown barley plants were exposed to infection by powdery mildew at different growth stages and for different periods of time. Mildew attacks up to G.S. 5·0 (Feekes scale, Large, 1954) greatly reduced the number of fertile tillers at harvest and also reduced the number of grains per head and grain size, even when the disease was controlled during later stages of growth. Mildew infection after G.S. 5·0 still reduced tiller numbers but to a lesser extent and yield loss at these later stages was due mainly to loss in grain size and grain number. In one experiment the size of the root system was recorded at harvest. Mildew, particularly when the infection occurred early (before G.S. 7·0), reduced the size of the root system. Root weight per plant was strongly correlated with grain yield per plant and tiller number per p