Think you don't have soybean cyst nematode (SCN)? Think again! Over 80% of the soybean fields in Illinois are infested with SCN. We have found SCN in 102 of 102 counties.
In high-yield environments, SCN can reduce yields up to 30% without causing any symptoms you can see. When you do see symptoms (such as yellowing and stunting), they're associated with either very high SCN numbers or some other problem in the field, adding to the stress caused by SCN. Common problems that show up in SCN-infested fields include Phytophthora rot, rhizoctonia root rot, and nutrient deficiencies. These can mask the underlying SCN problem. SCN infestation can also increase the likelihood that other diseases, such as sudden death syndrome (SDS) and brown stem rot (BSR), will show up later in the season.
SCN damage can be controlled by rotations with nonhost crops (such as corn) and the use of SCN-resistant varieties. To use rotation effectively, you need to know two things:
- Is SCN present in the field?
- What is the level of infestation?
To find out if SCN is present, you can do one of two things:
- Take a soil sample (directions following) and submit it to a lab for a cyst count.
- Wait until 40 to 50 days after planting soybeans. GENTLY dig up plants scattered throughout the field and GENTLY shake them to remove excess soil. Inspect the roots for small, white-to-yellow, glistening bodies on the surface of the roots (a hand lens makes this task much easier). These bodies will be much smaller than the nitrogen-fixing nodules. (See photo.)
To find out the level of infestation, you should submit a soil sample for an egg count. (Cyst counts will also tell you the level of infestation but are not very accurate if resistant varieties have been grown in the field.) It is extremely important to take a "good" sample (description following) because SCN populations are not distributed evenly in a field: If you sample in the wrong place, your count may be too high or low.
A soil sample should be a composite of 20 or more cores taken in a zigzag pattern across a field. Most nematologists agree that one sample can adequately represent a 5-acre area; some say as high as 20 acres. So what do you do if the field is 300 acres? Collect samples from two or more arbitrarily selected 5-acre sections that represent similar soil types and crop histories. There's no need to sample the entire field unless you're planning to plant different varieties in different sections of the field.
You'll need the following items: a soil-coring tube (you can use any soil-sampling device such as a trowel or shovel if a soil-coring tube is not available); a bucket; quart-sized Ziploc bags, one for each sample; a permanent marker; and a cooler. Collect about 20 soil cores (or scoops) to a depth of 8 to 10 inches from the sample area (see the previous paragraph for a description). Put each core into the bucket; and after all cores have been collected, mix thoroughly. Remove enough of the mixed sample to fill a plastic bag. Seal the bag and mark it with the permanent marker so that you'll know where the sample came from when the results come back. Place the sample in the cooler and keep it cool until you can pack it and ship it to a lab for analysis. Allowing the samples to heat up will effectively cook the nematodes in the soil and make it impossible for you to get a good analysis. If you're going to keep the samples for more than a day, put them in a refrigerator.
Illinois has a number of soil-testing labs that can do SCN analyses, both egg counts and cyst counts. In addition, the University of Illinois Plant Clinic can provide nematode analyses for any crop or home landscape plant (http://plantclinic.cropsci.uiuc.edu/).
For further information, call Terry Niblack at (217)244-5940 or email firstname.lastname@example.org.--Terry Niblack