Powdery mildew disease of suger beet has been known for many years in Europe and the Middle East, but it was seldom seen in California before 1974. The sudden and widespread occurrence of the disease in 1974 throughout the western United States is difficult to explain but probably was due to the introduction or development of a virulent race of the pathogen in or near the Imperial Valley, from where it rapidly spread by windblown spores to other areas. The continued presence of the disease in all areas in 1975 and 1976 indicates that a pathogenic race of this fungus disease is here to stay.