A recurring question often asked concerning plant-parasitic nematodes that feed on corn is: "When should soil samples be collected to check for these pests?" The answer depends on the details of each specific field situation. In most cases, fall sampling to test for nematodes that feed on corn is not recommended, says Greg Tylka, Iowa State University Extension nematologist. He offers the following observations and information about controlling corn nematodes.
Most nematode species present during early-to-mid season
The reason that fall sampling generally is not recommended is because to determine if nematodes are causing or have caused damage to a corn crop, the species and population densities (numbers) of the nematodes in a soil sample are compared to damage thresholds. But the numbers of most nematode species that feed on corn tend to decline as the corn crop matures and dies.
So low numbers from samples collected in the fall may be because the nematode numbers weren't very high (and not damaging) during the growing season or may be the result of numbers declining from a higher, possibly damaging level earlier in the season. There's no way to tell exactly what occurred during the growing season based on low nematode numbers in samples collected in the fall.
To determine if plant-parasitic nematodes are causing damage to corn, soil samples for nematodes should be collected early to mid season, when nematode numbers are generally greater. ISU recommends collecting samples whenever symptoms of damage, such as stunting, yellowing of foliage, mid-day wilting, lack of fine roots, swollen roots, and/or dead areas on roots are observed.
There are a few instances when fall sampling is warranted or even preferred, namely for root-lesion and lance nematodes and for needle and sting nematodes, as described below.
Root-lesion and lance nematodes are a special case
These two nematode species are endoparasites that enter corn roots and feed and reproduce almost completely within the roots throughout the growing season (see the accompanying photo). Early or mid-season soil sampling is acceptable for the root-lesion and lance nematodes. But fall sampling for these nematodes also is acceptable because nematode numbers accumulate in root tissue throughout the season. Root-lesion and lance nematodes need to be extracted from both soil and root fragments in order to accurately determine the population densities of these nematodes.
Sting and needle nematodes are found in sandy soil
These two nematode species are among the largest plant-parasitic nematodes, and their distribution is limited to soils with 70% or greater sand content. These nematodes reportedly migrate deep into the soil profile during the middle of the growing season and thus, they may be missed in soil samples collected mid season, even if soil cores were collected to a depth of 12 inches. The damage threshold for these nematode species is very low (one worm per 100 cm3 or about a half-cup of soil) and their numbers do not get very high.
So it is important to collect samples when there is the greatest likelihood of detecting low numbers of these nematodes. If nematode damage to corn is suspected in fields with high sand content (over 70%), spring or fall soil sampling for needle and sting nematodes is warranted.
Guidelines for collecting soil and root samples to check for plant-parasitic nematodes that feed on corn were discussed in an ICM News article July 19, 2010. More information about the different species of nematodes that can feed on corn was published in ICM News August 3, 2009.