If you see yellow leaves, stunting, or midday wilting in your cornfields next summer, the problem could lie beneath the soil. Microscopic worms (or nematodes) could be the culprit. The only sure way to find out is to collect soil samples and send them to a lab for diagnosis (see "Sampling for nematodes"). Or you may use yield maps and field histories to identify and treat potential hot spots.
Corn nematodes weren’t a problem from the 1950s to the 1980s. Soil-applied insecticides were used and some of them had activity on nematodes. This changed in the 1990s with the advent of transgenic traits for in-plant control of pests. Soil-applied insecticide use dropped dramatically. In the meantime, nematodes have been largely ignored, reports Greg Tylka, plant pathologist, Iowa State University.
One reason for this is because yellow or stunted corn can be caused by other factors like insect pressures and soil compaction, according to Keith O’Bryan, agronomy research manager, Pioneer Hi-Bred.