Delaying the date of sowing of spring‐sown barley in 1953 and wheat in 1954 from February to April increased the incidence of powdery mildew,Erysiphe graminisDC., from May onwards. Before then, conditions did not favour the rapid spread of mildew.From mid‐June, infected barley produced necrotic lesions. These developed sooner on the early‐ than on the late‐sown crops, and on the lower than upper leaves. Perithecia were first seen on 29 June. They were abundant on the wheat by 20 July.Mildew reduced the yield of barley sown on 30 March and 28 April 1955 by 8 cwt./acre. The percentage loss was greater in the late‐ than in the early‐sown crops. The number of ears per metre of row, the weight of 100 ears and the 1000‐grain weights were reduced. Altering the seed rate from 1 ½ to 2½ bushels/acre did not affect the incidence, or the effect, of mildew