The most accurate method for detecting nematode infestations is soil sampling [3, 4, 30]. A nematode analysis will determine the genera of nematode in a field and will provide an estimate of the population densities of each genus. Collect soil samples when soil moisture is adequate for good plant growth, not during dry periods. Irrigation may be necessary to have sufficient soil moisture for sampling.
Divide each field into 10- to 25-acre sections with uniform soil texture and cropping history to provide an accurate representation of the field. Collect soil with a 1-inch diameter probe to a depth of 8 inches within the crop root zone. Collect at least 10–20 soil samples in an arbitrary manner across each section. Thoroughly mix the soil samples from a section and put approximately one pint in a plastic bag. Seal the bag to prevent drying and label it to identify the field location. Keep the composite soil samples out of direct sunlight and heat. Place samples in a cool ice chest for transport to a state or private diagnostic laboratory.
Diagnostic services that identify nematode genera are available at most land grant universities and at multiple private laboratories in crop production regions. The nematode population levels, which vary widely between regions and soil types, can be compared with established economic threshold numbers for a specific crop in each state. The economic threshold is the nematode population density at which the value of the crop damaged is greater than the cost of the nematode-control method. Thus, nematode-control methods have an economic return.
Populations of plant-parasitic nematodes exhibit an uneven distribution across the field. Numbers often range from high to low or undetectable in different sections across a field. For this reason, if an area is suspected to have a nematode problem, keep samples collected from the area separate from other samples [4, 30]. Studies have shown that the reniform nematode is distributed evenly in conventional tillage systems [11, 14].
The time of year significantly influences nematode population densities. Nematode populations are generally at their maximum levels when the crop is at its greatest biomass stage. Thus, samples are usually collected immediately after harvest. Soil samples collected in the late winter after frost or in the early spring often contain low or undetectable nematode levels as populations decline with cold weather.