Figure 1. Wisconsin counties in which SCN has been detected. Click on image for larger version. (Source: Wisconsin Department of Agriculture, Trade and Consumer Protection, Cooperative Pest Survey Program and A. MacGuidwin, UW-Madison)

Historically, SCN in most states has gone from essentially undetected for years to a widespread problem leaving people wondering where it came from. The truth is that SCN was simmering for years in a stealth mode, even though people could have detected it years before it came a major production problem. Our goal in Wisconsin is to prevent a repeat of history which has occurred in neighboring states. It is not a case of if, but it is a case of when SCN damage will be more evident and widespread in Wisconsin.

SCN Risk Assessment
As of Fall 2006, the soybean cyst nematode has been found in 42 counties in Wisconsin, as indicated by dark shaded areas in Figure 1. In many cases, fields were found to be infested with SCN by random surveys rather than poor growth of plants or other symptoms of SCN damage.

Even in states with a long history of SCN, the pest does not cause dramatic symptoms and yield loss, but rather essentially undetectable symptoms and yield loss in the 3-5 bu/a range. This situation is often referred to as "hidden SCN damage".

Short soybean rotations increase the risk of yield loss from the soybean cyst nematode (SCN). The greatest yield loss occurs on sandy soils, but significant loss can occur on heavier soil types. However, the nematode is difficult to detect in loam to clay loam soils because symptoms are not as apparent compared to SCN activity in sandy loam soils.  SCN can not reproduce without a host plant, and can infect other crops such as edible beans. The soybean cyst nematode is moved every way that soil moves: water, wind and farm equipment.

Figure 2. White female cyst nematodes and cyst on roots of soybean. Note much larger N-fixing root nodules. Click on image to view larger version. Photo © Craig Grau.

Symptoms
Symptoms of SCN infection are often patches of stunted, yellowed, less vigorous plants. However, SCN can cause significant yield loss without causing obvious aboveground symptoms.

The only definitive symptom of SCN infection is the presence of adult female nematodes and cysts on the soybean roots (Figure 2) . Roots infected with SCN are often dwarfed or stunted, with a decrease in the number of nitrogen-fixing nodules on the roots. Be sure to compare to a known healthy root system.