Sampling for corn nematodes
Nematodes are microscopic roundworms that attack roots of corn and other plants. In corn, nematodes affect yield by damaging the root system and interacting with opportunistic root-rotting fungi. They become more important when there are other root-related stresses in the same field, such as moisture shortage, root damage from insects, nutrient deficiencies, or compaction.
Although there is increasing interest in nematodes as potential yield-limiting factors, the threat these pose varies greatly from one field to the next. The only way to determine whether nematodes may be affecting yield is to sample for nematodes.
When to Sample
The window at 4-6 weeks after planting is probably the best time to sample, since the nematodes will be in the root zone and the soil is often moist enough to be able to insert a soil probe. Plus, sampling early in the growing season will give an idea as to whether nematodes are likely to affect crop productivity. Summer sampling is less than ideal. During summer, certain nematodes (like the sting nematode and needle nematode) move downward in the soil profile, where the moisture is located. Also, pulling samples from dry soil is difficult. Sampling after harvest has its advantages, since it gives a look at certain nematode populations, lesion nematode, which burrow within the corn root), as well as the nematodes that go deep into the soil during summer.
How to sample
The most important thing to realize is that sampling for corn nematodes is different than sampling for soybean cyst nematode. For corn, sampling both soil and roots is advisable for best results. Sampling guidelines are as follows:
• Instead of sampling at random or zig-zag pattern (like you would for soybean cyst nematode), sample parts of the field where yield losses or symptoms (stunting, yellowing in elongated areas of the field) are not explained by other factors, such as soil compaction, soil type, etc. Corn nematode populations can be extremely variable (see Figure 1), so focus sampling in problem areas of the field. However, if some of the corn is severely affected, sample from the edge of the damaged areas rather than in the worst areas. It is important to avoid badly damaged corn, because corn nematodes require live roots to feed on, and if plants are severely damaged or dead, the nematode numbers will be low.
• Make sure to sample within the row—that’s right, within the row. This is different from sampling for soil fertility. Take 20-25 samples.