Spatial pattern analysis of Heterodera glycines population densities in experimental settings to soybean variety evaluations in the United States

In the U.S., hundreds of soybean varieties are evaluated each year for agronomic performance and response to the soybean cyst nematode (SCN; Heterodera glycines), the most economically important pathogen of soybean. Researchers generally recognize that in field experiments the analysis of the spatial variability of nematode population densities can be an important tool to enhance sampling schemes and to interpret nematode population dynamics. Yet, in experimental settings to evaluate SCN-resistant soybean varieties, how individual SCN sampled entities are arranged within and among plots remains unknown. In this study, we characterized the intra- and interplot variability of SCN population densities in three experimental areas using intensive mapping through composite and single-core cluster sampling. The β-binomial distribution adequately described data of incidence (number of SCN-positive soil cores per plot) and suggested that SCN population density aggregated within plots in the three locations (index of aggregation θ > 0). Correlogram analysis of 36 transects of 28 and 35 m showed that the interplot variability of SCN population densities extended up to 15 m away from developed foci. Altogether, the results suggest a varying probability of recovering cysts within plots during sampling and an uneven pathogen pressure on soybean plants within plots. This prompts the need to consider the spatial dependence of SCN population densities in variety evaluations, especially when rigorous variety comparisons are desired.