Powdery mildew of sugarbeet, caused by Erysiphe polygoni DC., was first reported in California in 1934 but did not become prevalent statewide until the 1974 season. Observations in Imperial County show wide differences in intensity of powdery mildew in various fields. In some sugarbeet fields many plants were completely covered with mildew while others had medium to light infestation of leaves. A question was raised as to whether it was necessary o treat every field in the valley for control of powdery mildew. To help answer this question a sulfur-dust airplane trial was initiated in the 1974-75 growing season.