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Effect of Secondary Selection on Cyst Nematode Reproduction on Soybeans¹

"Resistance" of soybeans [Glycine max L. (Merr.)] to soybean cyst nematodes (SCN, Heterodero glycines Ichinohe) is determined by the relative number of nematode females or cysts that can develop in the roots of soybeans. Soybean cyst nematode (SCN) populations have been selected for ability to reproduce on soybeans with several sources of resistance and were used to identify genetic differences in both SCN and soybeans. The objective of this experiment was to determine reproductive changes in SCN populations initially selected on ‘Cloud’ or on ‘PI 89772’ after secondary selection on the other, genetically-different, source of resistance. Response to the secondary selection was rapid; apparently SCN genes for reproductive ability were not fixed by the primary selection. Four cycles of selection or SCN generations on Cloud, of the population initially selected on PI 89772, significantly increased its ability to reproduce on Cloud while concurrently decreasing it on PI 89772. When the SCN population initially selected on Cloud was then selected on PI 89772, it could reproduce relatively better on PI 89772 but less well on Cloud. Thus, SCN genes for ability to reproduce on specific soybeans are not independent. Williams, the "susceptible" check, may have at least one of the same genes for resistance as Cloud. Evidence from SCN populations selected from four other field populations indicates that at least some of the SCN genes for reproduction on genetically different soybeans probably are alleles. By judicious deployment of the soybean genes, we may be able to keep SCN populations below damage-threshold levels.

Key Words: Glycine max L. (Merr.) • Heterodera glycines Ichinohe

² Res. Agron., USDA, ARS, Dep. of Agron., and Prof., Dep. of Plant Path., Univ. of Missouri, Columbia, MO 65211.

Received for publication March 25, 1982.