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Fall optimum time to test for SCN

By Staff | Sep 26, 2011

“Compare the results ... after every third or fourth soybean crop to know (if) numbers are increasing.” —Greg Tylka ISU nematologist

By Darcy

Dougherty

Maulsby

Farm News staff writer

As the combines roll, fall offers the perfect time to collect soil samples, especially for soybean cyst nematodes, which rob farmers of millions of dollars in lost soybean yields each year.

Testing for SCN in the fall after harvest can help growers identify SCN-infested fields and begin managing it before damage becomes severe.

“Preventing increases in SCN population densities is necessary for continued profitable soybean production, because SCN is extremely long-lived in the soil,” said Greg Tylka, an Iowa State University Extension nematologist.

While SCN can move through the soil only a few inches per year on its own power, it can be spread substantial distances in a variety of ways.

Anything that can move infested soil (even small amounts) is capable of spreading SCN, including farm machinery, vehicles and tools, wind, water, animals and farm workers.

Unlike other soybean pests, SCN can cut yield without having any visible symptoms. In other cases, yellow spots may appear in soybean fields in late July or early August.

While farmers often assume the yellowing is due to iron chlorosis, it may be SCN.

“SCN usually is present in fields for many years before population densities increase to a level that causes obvious stunting or yellowing,” Tylka said. “That’s why many healthy-looking fields are actually infested with SCN.”

While growers can test for SCN any time, in the fall after harvest is best for three reasons including:

  • Poor yielding areas are fresh in mind. Whole fields or field areas with lower yields that can’t be explained by soil type, flooding, weed pressure or other factors that can reduce soybean yields are commonly caused by SCN.

This is especially true if the same areas have good corn yields, but poor soybean yields. Concentrate soil sampling in these areas.

  • You can order SCN-resistant seed, if necessary. Resistant varieties and crop rotation are the two main components to managing SCN.

If a growers detect SCN this fall, they can still adjust seed orders.

“If you have SCN, switching to resistant soybean varieties could pay dividends to the tune of 10, 15, 20 or more bushels per acre in yield,” Tylka said.

  • Farm activity slows down after harvest. Taking soil samples can be the most valuable time growers spend in their fields all year, if they look at the potential return.

Stopping the spread

The key to profitable long-term soybean production in SCN-infested fields is to prevent SCN population densities from increasing.

“Compare the results of soil samples collected and analyzed for SCN population densities in the fall after every third or fourth soybean crop to know whether SCN numbers are increasing,” Tylka said.

Comparing results of soil samples collected six to eight years apart requires good record keeping and also consistent soil sample collection methods.

Accurate and detailed notes of when and how samples were collected are needed so the same methods can be used in future years.

Details should include the specific areas of fields that are sampled, the numbers of cores that are collected and their depth, the specific sampling date, whether samples are collected before or after a soybean or other crop, and which laboratory processes the samples.

Tylka added the following points:

  • Collect soil samples in the fall after the previous crop has been harvested or in the spring, before the new crop is planted.
  • The more soil cores collected and the smaller the area sampled, the more accurate the results will be.
  • Soil cores should be from the upper eight inches of soil.
  • If corn or some other non-host crop was last grown in the field, it doesn’t matter if soil cores are collected in the previous crop’s row.
  • It is better to collect soil cores after the previous corn or other non-host crop’s rows have been destroyed by tillage.
  • If soybeans were last grown in the field, collect soil cores from under the old crop rows.

You can contact Darcy Dougherty Maulsby by e-mail at yettergirl@yahoo.com.

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