Relationships among tillage, spatial patterns of Heterodera glycines, and soybean yield
Heterodera glycines spatial patterns were studied under different tillage systems in naturally infested soybean fields from 1994 to 1997. The tillage treatments were conventional tillage, reduced tillage, ridge tillage, and no tillage. Results of geostatistical analysis of both cyst and egg population densities showed that no tillage and ridge tillage promoted aggregation of the nematode population, but conventional and reduced tillage resulted in a more uniform spatial pattern. The relationship of soil properties and H. glycines population densities were investigated in the no-tillage plots. Nematode population densities were positively correlated with soil pH, and negatively correlated with organic matter content. Generally, correlations of nematode population densities with K and P were not significant;A second study was conducted from 1995 to 1998 to determine the impact of tillage practices on H. glycines dissemination, reproduction, and yield impact on soybeans. In one experiment, treatments were no tillage, non-infested; no tillage, H. glycines-infested; ridge tillage, H. glycines-infested; conventional tillage, H. glycines-infested; and reduced tillage, H. glycines-infested. After 1 year, nematode population densities were lower in the infested, no-tillage treatment compared to infested, conventional- and reduced-tillage treatments. After 2 years, H. glycines had been disseminated by conventional and reduced tillage at farther distances compared to no-tillage and ridge-tillage treatments. Soybean yield decreased in all treatments over time, but the decrease was more pronounced in the conventional tillage treatment. A second experiment, was established in 1996 to evaluate the effect of spatial patterns of H. glycines populations on yield of soybeans. There were three treatments: non infested, aggregated infestation, and uniform infestation. There were no significant differences in soybean yield among the three treatments in 1996, 1997, and 1998. Heterodera glycines was rapidly disseminated by tillage in newly infested fields, facilitating more rapid nematode reproduction and subsequent yield loss. Yield reduction in tilled, infested plots may have been due to a combination of greater dispersal of H. glycines and other unidentified effects of tillage on the nematode.