Check Fields For Soybean Cyst Nematodes

John Wilson
Extension Educator
Loren Giesler
Extension Plant Pathologist

Now is a good time to start checking fields for soybean cyst nematodes (SCN) when scouting for other insect or disease problems. Cysts will develop on soybean roots about a month after the soybeans emerge. If soybeans were planted by late May, cysts should be developing on soybean roots in infested fields.

The cyst is the only stage in the SCN life cycle that can be seen without a microscope. Look for a small, lemon-shaped, white- to cream-colored "bump" on the outside of the root. These can be confused with the nodules which contain the nitrogen-fixing bacteria that are normally found on a soybean root.

The cyst is much smaller, oblong, and is lighter in color than nodules. When scouting fields, if you don't find any cysts, it doesn't necessarily mean your field is SCN-free. However, if you do find cysts, you'll need to act to reduce its buildup in the soil and resulting yield loss.

Soil Sampling for SCN

To more definitively determine whether you have SCN, you need to take a soil sample, much the same as you would take a top soil sample for fertilizer recommendations. Take 15-25 cores from a field, mix them together, and then take your sample from this mixture. The Nebraska Soybean Board has partnered with UNL Extension to provide bags to submit soil samples for a free SCN analysis, a $20 value. These are available at your local UNL Extension office.

Figures 1a-d. Lemon-shaped SCN cysts on soybean roots

Multiple Generations

The cyst is the female nematode that lived inside the soybean root. As it develops eggs, it swells up and ruptures through the root wall. Each cyst can contain up to 400 eggs. These eggs are released into the soil and the life cycle repeats in another month.

Because SCN can have several generations each growing season and because it is a prolific egg producer, the population of SCN can build dramatically in a field in one season. Even though the SCN population may not be high enough to cause yield damage this year, it can build up to damaging levels for the next time soybeans are planted, even if there is a year of corn between.

Managing SCN

The last eight years, we had 29 research sites comparing SCN resistant and susceptible soybean varieties in fields infested with SCN and 11 sites where the same varieties were planted in fields with no SCN. In the infested fields, resistant varieties outyielded susceptible varieties by an average of almost six bu/ac (an 11% increase). In fields where SCN was not present, there was no significant difference in yield between susceptible and resistant varieties.

In almost all of the plots on infested sites over the years, you could not distinguish between susceptible and resistant varieties. Susceptible varieties were not yellow or stunted and all plants looked healthy. That is why it is so important to scout for SCN now. Many producers are not aware that they have fields infested with SCN.

With other pests, it is easy to see damage such as shot holes from corn borer, plants cut off from cutworms and leaf or pod feeding from bean leaf beetle or grasshoppers. However, with SCN, there may be no visible damage.

Frequently, the first indication of a problem is at harvest. Soybean yields hit a plateau or drop back for no apparent reason, while corn yields continue to improve. This doesn't confirm SCN in the field, but it would be one of the first things I'd check.

And if you're thinking about putting off an SCN check for another season, do the math. A six-bushel-per-acre yield increase at today's prices could put a lot of money in your pocket at the end of the season by managing for SCN.  

Source : unl.edu