How do I know if I have the soybean cyst nematode in my fields?
In Minnesota, SCN has been found in most (64) soybean-growing counties. However, 50% of soil samples near-randomly collected from soybean fields throughout the soybean-growing area in Minnesota in 2007-08 were not infested with SCN or had undetectable low SCN population densities. While most fields in southern Minnesota are infested by SCN, a large proportion of fields in northern Minnesota may have no or low SCN infestation.
Early detection is important for managing SCN and minimizing yield loss to the pest. To determine whether your fields have an SCN problem and how severe it is, you may need to look for any plant symptoms in the field, scout females (cysts) on roots, monitor soybean yield, and/or have soil samples tested for the presence of SCN eggs.
Stunting and chlorosis are typical symptoms of soybean induced by SCN (See Section 4), however, SCN can cause yield loss in the absence of visible symptoms. Declining yields from a field or portion of a field are sometimes the first clue that SCN could be causing a problem. Subtle signs such as areas of uneven soybean heights, slow row closure or expanding, or out of place nutrient deficiency symptoms may also be clues to SCN infestation.
The most accurate diagnosis of an SCN problem is to find the nematode on plants or in soil. The unique, diagnostic sign of SCN infection is the living mature female nematodes or cysts attached to roots. These tiny, lemon-shaped, white to yellow females usually can be seen on the roots beginning 4 to 5 weeks after planting. The cysts on roots are usually abundant in July and August and then decline in numbers as roots senesce. Visible females on the roots increase and decrease as generations of SCN are produced.
Plant roots to be examined for the presence of females need to be gently dug rather than pulled from the soil to prevent loss of the cysts. Gentle rinsing of soil from the roots in a bucket of water will help reveal their presence. Adult females and cysts are about 1/40 inch long and 1/60 inch wide and are large enough to be seen with the unaided eye (Fig. 8). They are easily distinguished from the much larger bacterial nodules on the roots.
Females may be difficult to find on roots when the SCN population density is low, when sampling is done too early or too late in the growing season, or when the SCN population density is extremely high. In the latter case, soybean roots that are severely damaged due to the actions of the nematode and associated microorganisms will no longer be capable of supporting SCN. Under such circumstances, analysis of soil samples from suspect fields by a professional laboratory may be necessary to detect the presence of SCN.