Abstract

Studies of soybean [Glycine max (L.) Merr.] cultivar sequence and use of nematicides were conducted on a soil uniformly infested with the soybean cyst nematode (SCN), Heterodera glycines Ichinohe. Assumption that new races of SCN would develop rapidly if only SCN resistant cultivars were grown had led to specific recommendations that SCN resistant cultivars not be grown > 2 yr without rotating with a SCN susceptible cultivar or a nonhost crop. No field data were available to support the recommendation. This study was initiated in 1976 to more critically evaluate the effects of SCN on soybean productivity in a long-term monocropping program. Resistant to SCN race 3, ‘Centennial’, and one that is susceptible, ‘Tracy’, were grown continuously with and without a nematicide. The nematicide treatment was dropped after 6 yr. The two cultivars were also grown in a sequence of 2 yr of Centennial and 1 yr of Tracy and in a blend of 80% Centennial with 20% Tracy. A breeding line resistant to SCN races 3 and 4 constitutedhn additional treatment. Sorghum [Sorghum bicolor (L.) Moench.] replaced the nematicide treatment in the seventh or seventh and eighth years to be followed by Tracy. Nematicide treatment did not enhance seed yield for either the susceptible or the resistant cultivar. No treatment was superior to growing the SCN race 3 resistant cultivar Centennial continuously over the 10-yr period. No advantages were determined for including a SCN susceptible cultivar in the program. Beginning with the third year of this study and seventh year of continuous soybean production, the SCN population appears to have stabilized at a low level.

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