Sampling Depth and Crop Growth Stage Affect Estimates of the Population Density of Lesion Nematodes (Pratylenchus spp.) in Corn Fields in Ohio

The effects of sampling depth and crop growth stage on the population density of lesion nematodes were investigated in three commercial fields in Wayne and Fulton Counties, Ohio, during the 2016 and 2017 growing seasons. Soil samples were collected at five growth stages by removing 15 soil cores to a depth of 70 cm from each of 25 plots per field-year. Cores were divided into seven 10-cm sections, and nematodes were extracted from the soil and root fractions of each of them. Pratylenchus crenatus and P. thornei were detected in approximately 84 and 78% of the samples collected in Wayne and Fulton Counties, respectively. Depth significantly affected total population density of both species as well as densities in the soil and root factions in all field-years, but the effects of growth stage and its interaction with depth varied with field-year. In most cases, mean population densities were higher from 10 to 40 cm soil depth than at the reference 40 to 50 cm depth and lower from 50 to 70 cm. There were quadratic relationships between population density (on the log link scale) and depth, with the highest peaks in estimated predicted densities generally occurring between 20 and 40 cm, depending on crop growth stage and growing conditions. These findings suggest that the standard practice of sampling between growth stages V3 and V6 to a depth of 45 to 50 cm and using the entire core for extraction and enumeration could lead to underestimation of population densities of P. crenatus and P. thornei.