In the cultivation of soybean plants in Hokkaido and in the Tohoku region of Japan, the most difficult problem at present is soybean dwarf virus disease (SDV). This disease was first found on the soybean variety 'Tsurunoko' in southern areas of Hokkaido in about 1952. Since then researchers have been attempting to overcome it, and some aspects of this disease were clarified: for example, this disease was not transmitted by manual inoculation of sap or through seeds, but was transmitted by the grafting method or foxglove aphid in a persistent manner. Two strains of SDV were obtained from naturally infected soybean plants.