Heterodera glycinesPopulation Dynamics and Relation of Initial Population to Soybean Yield

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Abstract
Soybean (Glycine max ''Forrest'') was grown in monoculture and in a 2-, 3-, and 4-yr rotation with corn (Zea mays), cotton (Gossypium hirsutum), and grain sorghum (Sorghum bicolor). The effects of these crops and rotation lengths on soybean cyst nematode (SCN) (Heterodera glycines) population dynamics and soybean yield responses were measured. Annual and oversummer decline of cysts and eggs in the absence of soybeans, overwinter decline of eggs, and population increase when soybeans were grown were dependent on the initial population density. Overwinter survival of cysts was density-independent. There were no significant differences among rotational crops in their effects on SCN population dynamics and soybean yield. Two and 3 but not 1 yr in a nonhost crop resulted in significant soybean yield increases over yields obtained from monoculture. SCN population level were at or near the limits of sampling detection after 3 yr of a nonhost. The Seinhorst equation estimated a tolerance threshold at 3 cysts or 470 eggs per kilogram of soil.