Soybean cyst nematode (SCN) is a small, plant-parasitic roundworm that attacks the roots of soybeans. SCN is widespread throughout Iowa and surrounding states, and can cause as much as 40% yield loss without symptoms appearing on the soybean plants.
Symptoms to Watch For
A recent Wallaces Farmer article highlighted what symptoms to watch for when checking for SCN in your field. Symptoms are usually not apparent when temperatures are moderate and rainfall is adequate to excessive in the growing season. Symptoms generally begin to appear in mid-July and last through the growing season, and include stunting of plants, yellowing of foliage (not just leaf margins or areas between leaf veins) and mid-day wilting.
Dig, Dig, Dig
The only way to check for SCN is to dig roots, gently remove soil from the roots, and look for the egg-filled, round, white SCN females on the roots (see image). Females are about 
the size of the head of a straight pin or a period at the end of a sentence in a magazine – for most people – females can be seen with the unaided eye. SCN females should be apparent on infected roots through August.
While this is an effective way to identify SCN, it’s also an effective way to see which soybean varieties are controlling nematode reproduction in fields known to be infested. If a soybean variety is effectively controlling the nematode, there should be only 10-20 SCN females present on the roots.
Click here for the full article.