The foreign and domestic germplasm collections of soybean were tested for their resistance against soybean dwarf virus (SDV) under natural infection at Hokkaido Central Agricultural Experiment Station. Among the 2,075 soybean varieties tested, twenty were resistant to SDV though their resistance was not complete. As the agronomic characteristics of these resistant varieties are inferior to those of the cultivars recommended in Hokkaido, artificial crosses between these resistant varieties and the recommended cultivars have been performed since 1971. Hitherto, attempts have been made in 134 pairs of crosses to breed SDV resistant varieties having superior agronomic characteristics. We were able to select about 30 promising lines which showed a low percentage of SDC infection, good plant type, early maturity and good quality in the early generations. In the middle generations, evaluation of SDV resistance in the selected lines was confirmed under natural infection in Date city where SDV usually occurred very seriously. Moreover, the selected promising lines were also evaluated by aphid inoculation tests, using viruliferous aphids. As a result of the soybean breeding program mentioned above,
we eventually selected 5 promising lines and one of them, the new cultivar 'Tsurukogane' was released in the southern districts of Hokkaido in 1984. This cultivar which is highly resistant to SDV displays desirable agronomic characteristics such as high yield and good quality.