Plant-parasitic nematodes are microscopic worms that primarily live in the soil and feed on the roots of plants, including corn and soybean. More than 70 percent of Iowa’s fields are thought to be infested with a particularly damaging nematode, the soybean cyst nematode (SCN). And most every field in the state harbors one or more nematode species that can feed on corn. Most nematodes are not thought to be damaging to corn until their numbers increase to high levels. Now is the time of the season when obvious damage from nematode feeding on corn and soybeans is likely to become apparent.
Two very damaging nematodes that feed on corn are the needle and sting nematodes, and they severely stunt the growth of plants and cause obvious leaf yellowing very early in the season (Figure 1). But needle and sting nematodes only occur in soils with at least 50 percent sand content, so they are not of great concern in most Iowa fields.
Most other nematodes that feed on corn are present at low population densities in the beginning of the season, then their numbers can increase to damaging levels, but this buildup takes many weeks. Symptoms of damage from these nematodes include stunting and foliar yellowing (Figure 2), and the symptoms typically begin to appear in July and August. Guidelines for checking stunted and/or yellowed corn for possible nematode damage was reviewed in the June 2011 ICM News article, Sampling for Nematodes that Feed on Corn. There is no reason to check for damaging population densities of plant-parasitic nematodes on corn if symptoms are not occurring.
SCN has been recognized as a serious yield-limiting pest of soybean in Iowa and throughout the Midwest since the late 1980s. This nematode was a high-profile pest in the 1990s and 2000s, but interest and concern about SCN have waned in the past several years. Soybean yields can be reduced by 30 percent or more due to SCN without obvious above ground symptoms appearing, and significant yield loss (5 to 10 bushels per acre) can occur even in years of adequate to excess rainfall. Also, dormant SCN eggs can survive in dead females, called cysts, in the soil for a decade or more. Consequently, fields are vulnerable to yield loss from SCN every year.
When above ground symptoms of damage from SCN occur, the soybeans are stunted and yellow (Figure 3) and rows are slow to close over. At times, these symptoms can be very severe. The symptoms typically appear mid to late July or later and occur more frequently when soil moisture declines in the second half of the growing season. Fields can be checked for the presence of SCN by looking for white SCN females on soybean roots at this time of the year; guidelines for doing this were discussed in the June 2011 ICM News article, Females Now apparent on Soybean Roots.