Effect of Aphid‐Transmitted Soybean Mosaic Virus on Yields of Closely Related Resistant and Susceptible Soybean Lines1

The effect of soybean mosaic virus (SMV) resistance on soybean yields during three growing seasons (1974, 1975, and 1976) was determined under circumstances of natural virus transmission by using closely related SMV‐resistant and susceptible lines (27 pairs). They were developed by backcrossing SMV resistance derived from PI 96983 or PI 170893 to cultivars ‘Dare,’ ‘Semmes’, ‘Pickett 71’, ‘Lee 68’, and ‘Ransom’. Susceptible lines in two of four pairs from the Dare backcross family manifested a “field resistance”, i.e., no significant yield reduction (5.2%). Yield reductions in other backcross families averaged from 35 (Semmes) to 20% (Lee 68 and Pickett 71); Ransom was intermediate at 27%. The relationship of SMV infection to seed quality was examined by scoring seed from 1975 visually and by plating out seed from 1976 on agar to determine the percent infection by Phomopsis sp. Seed of susceptible lines appeared inferior to seed of resistant lines in 16 of 26 comparisons; in 17 of 26 pairs, Phomopsis sp. was isolated with greater frequency from SMV‐susceptible lines than from SMV‐resistant lines (18.7%) vs. 4.5%).