Interest in plant-parasitic nematodes as pathogens of corn is greater now than in the past 20 years.
Unfortunately, little research on the biology, scouting and management of nematode parasites of corn has been done since the 1980s, and there are no clear answers to many basic questions, such as when samples should be collected to scout for nematodes on corn.
Population densities (numbers) of most species of plant-parasitic nematodes that feed on corn increase through the growing season, then decline as the corn crop matures and eventually dies. And most species do not damage corn at low population densities.
Researchers in the 1980s established damage thresholds for the different nematodes that feed on corn, and the damage thresholds were based on “mid-season” population densities. It is not clear what specific calendar dates or corn growth stages were meant as “mid-season” by the researchers.
Research on nematodes that feed on corn is currently underway in many Midwest states, including Iowa. And recent work with the endoparasitic root-lesion and lance nematodes on corn indicate that the V6 growth stage is a good time to start to collect soil and root samples to assess population densities of nematodes.
It is unlikely that a healthy-looking corn crop would be growing in soil infested with damaging population densities of plant-parasitic nematodes. So sampling fields indiscriminately is not advised.
But it might be worthwhile to collect samples to test for plant-parasitic nematodes in fields of unthrifty corn, and sampling near the V6 growth stage seems to be a good time.
At this stage in the corn crop development, sampling should done be as follows:
• Collect 10 to 20 12-inch-deep soil cores from the root zone of unthrifty plants.
• Collect 5 to 10 root masses from V6 plants; the tops of the plants can be cut off and discarded, and soil adhering to roots can be removed as well.
• Place soil cores in a sealed plastic bag; roots can be placed in a separate plastic bag.
• Protect the samples from temperatures above 80 degrees, and do not be physically rough with the samples (by throwing them, for example).
• Deliver or send the samples to a laboratory for processing as quickly as possible; avoid shipping samples on Thursdays and Fridays so that samples do not sit in delivery trucks over the weekend.
Several private laboratories and most land-grant university plant diagnostic laboratories process samples and determine the identities and numbers of plant-parasitic nematodes present.
If damaging population densities of nematodes are found, there are no in-season tactics that can be implemented to mitigate the yield loss that will occur in the current growing season.
Primary management strategies for future years are use of soil-applied Counter 15G and 20G nematicides and/or seed treatments such as Avicta and Votivo. The information obtained from samples collected this year may pay dividends in the form of more profitable corn production in future years.