| Heterodera glycines Ichinohe 1955 (NEMATODA) (soybean cyst nematode, SCN) is recognized as the major pest limiting soybean [Glycine max (L.) Merr.] production, accounting for approximately 1.67 × 109 U.S. dollars of soybean yield loss annually in the United States. Despite current management efforts, SCN continues to spread throughout soybean producing areas worldwide. The goal of this project was to understand SCN spatial distribution in soybean fields as the first step towards developing site-specific management (SSM) strategies for SCN. If SCN is to be managed site-specifically its spatial distribution should be structured and relatively time invariant, and it has to be related to yield-limiting factors easier to monitor and manage. The literature suggests that SCN may meet these requirements for SSM. Geostatistical tools and classical statistics were applied to test the hypotheses that SCN's spatial distribution within a field is sufficiently structured and time invariant; that SCN spatial distribution and population densities are related to soil properties; and that the relations among SCN population density, soil properties and soybean yield are sufficient in magnitude to aid in the management decision-making process. A nested survey sampling design was applied on two SCN-infested fields in MI and soil and soybean root samples were collected at monthly intervals during the growing seasons of 1999 and 2000. Soil samples were analyzed for SCN population density, soil fertility and soil texture. The SCN population in the roots was also quantified. To assess host response, soybean leaf samples were collected twice in 2000 for tissue analysis, and soybean yield was recorded in 1999 and in 2000.; The within field variability in cysts, eggs per cyst, and eggs was large in both fields. The spatial structure in SCN population varied with sampling times, but a periodic pattern in semivariograms appeared consistently from planting to harvest in both fields. The difficulty in adequately fitting wave models to the empirical semivariograms underestimated in some cases the spatial structure in SCN population. Soil texture, pH, and calcium concentration in the soil were strongly correlated and cross-correlated with SCN density in the soil, and to a lesser extent in the roots. Correlations were maintained consistently over time. The nutritional status of the crop reflected the interactions of soil fertility, soil texture, and SCN population density. Bean yield was also strongly correlated with soil texture, soil pH and calcium concentration, and SCN population density in the soil.; The results contribute significantly towards the understanding of SCN spatial distribution in soybean fields, provide evidence of the underlying factors involved in determining grain yield, and lay the base for further research on cause and effect relations to advance understanding SCN biology, ecology, and management opportunities. The spatial variability in yield correlated to the combined effect of SCN density and unfavorable soil conditions observed in this work provided support for the notion of management zone delineation. Thus, the hypotheses tested were demonstrated true. |
