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An 'Art and a Science': Scout SCN Now to Prevent Yield Lag Later

It's a growing pest in a lot of soybean fields in the Corn Belt. Yet, its damage to crop potential doesn't show up until you run the combine in the field. That makes soybean cyst nematode (SCN) scouting -- something experts say is "part art and part science" an important task to add to your fall to-do list.

"Crisp, clear fall days are perfect for splitting firewood, tilling the garden under, and collecting soil samples to check fields for the soybean cyst nematode," says Iowa State University Extension plant pathologist Greg Tylka. "Although soil sampling for SCN might not be on most people’s list of favorite autumn chores, fall is a great time to sample fields for this pest. Reasons to sample for SCN include checking for the presence of the nematode in fields and monitoring SCN numbers in fields known to be infested with the pest."

The microscopic pest can carry a huge cost for soybean farmers; in Nebraska alone, farmers lost an estimated $45 million in crop revenue from SCN-induced yield losses in 2013, and the pest was found in 56 counties in that state this year. Now, SCN wasn't as bad there this year, says University of Nebraska Extension educator John Wilson. Still, because of the nature of the pest's damage, even light SCN pressure can hit your pocketbook hard.

"What makes SCN such a problem is that yields can be reduced 20% to 30% with no visual signs on the plant. Plants can appear normal and healthy but suffer significant yield losses," Wilson says in a university report. "In fact, often the first indication of an SCN infestation is a soybean yield that seems to hit a plateau or even start decreasing while corn yield in the field continues to increase."

Another reason to conduct good scouting in the fall: planting decisions next spring. If you've got just light SCN pressures now, you're probably OK with planting a SCN-resistant soybean variety next year. But if you've got a lot of SCN on your hands, it may mean a change to your crop rotation altogether, Tylka says.

"If SCN is detected at low or moderate population densities in fields slated for soybean production in 2015, growing SCN-resistant soybean varieties is recommended," he adds. "If SCN numbers are high, a second year of corn might be considered to decrease SCN population densities so that SCN-resistant soybean varieties can produce profitable yields when soybeans are again grown."

Sample an area no larger than 40 acres per sample (less is better).

  • Take a minimum of 20 to 25 soil cores from randomly selected areas. If the field has standing soybean stubble, take your sample just a couple inches to the side of the old soybean row and go 6 to 8 inches deep. (This way you'll be probing through the old root system and are more likely to detect SCN if it is there.)

  • If you are sampling in a field that wasn't in soybeans this year but will be going to soybeans in 2015, randomly collect samples from across the area to be tested. (If you find SCN, this information will be important when you order seed. When selecting an SCN-resistant variety, remember to also evaluate other traits you want, including emergence, lodging, or chlorosis ratings and resistance to other diseases, etc.).

Adds Tylka: "If possible, collect separate multiple-core samples from different areas or management zones in large fields. If grid sampling, collect one or two soil cores from every grid cell and combine cores from the number of cells that represent approximately 20 acres. Do not collect samples if the soil is muddy or frozen."

Now, for the "art" of effective SCN scouting . . . It starts with identifying areas where the pest is most likely to thrive, Wilson says.

"When selecting which areas within a field to sample, remember: Anything that will move soil will also move SCN, including wind, water, wildlife, humans, and equipment," he says. "Think about the areas in your field where soil may have been moved because these are areas where SCN is likely to have first become established. Take several cores in each of these areas."

Those areas, according to a university report, include:

  • Along a stream that periodically floods. An upstream field may have had SCN that washed down to your field.

  • Low areas where water drains after a heavy rain. Light infestations throughout the field may be concentrated in these low areas where water stands.

  • Along fence lines. In the past when fall tillage was more common, fences would act like a trap for the blowing soil and SCN.

  • By field entryways or driveways. This is the most likely place for soil from another SCN-infested field to travel in on equipment and be brushed off.

"Also include areas with a higher incidence of sudden death syndrome or brown stem rot. You can have either SDS or BSR without having SCN, or you can have SCN without having either of these diseases. However, if you have SCN in a field or even part of a field, you are more likely to have SDS or BSR in the affected area; areas where soybean yields are less than expected and there is no difference in soil type or incidence of soil compaction, herbicide injury, weed, insect or disease pressure, or other explanation for lower yields. This can be a whole field, but more commonly is an area in the field," Wilson adds. "In either of these situations, take a sample where a problem is suspected and one close by but outside this area. This can help you confirm or eliminate SCN as the cause in the suspect area."

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