It is easy to see when weeds or insects develop resistance to a herbicide or insecticide. The weeds or insects are easy to spot in the field. Even some plant diseases are becoming resistant to some fungicides, as evidenced by the infected plants in the field.
A harder place to detect resistance may be when soybean cyst nematodes (SCN) start to overcome the benefits of using a soybean variety with a particular source of resistance to SCN.
Of the hundreds of SCN-resistant soybeans available to producers, the vast majority use the same source of resistance, PI88788. The explanation for this is simple: This is the easiest source of resistance to breed in and still maintain high-yielding varieties. (This is done with traditional plant breeding techniques, not GMOs.)
For many years in many fields, those varieties did just what they were supposed to do. They limited SCN reproduction and allowed soybean growers to lower high SCN egg counts or to keep low counts low while maintaining yields.
Managing SCN is all about numbers. If you have SCN in your field, you will never be able to eliminate it., at least not with today's management options. Rather, the goal is to reduce that number as much as possible.
SCN populations are measured by egg density, the number of eggs in 100 cc's of soil. For a reference point, that's about enough soil to fill a pop can one-third full.
A certain portion of any SCN population in a field will reproduce on any source of resistance. There is no source of resistance that eliminates all nematodes from reproducing.
When the same source of resistance is used whenever soybeans are planted in a field, the same selection pressure is placed on those nematodes remaining in the soil. And just like when the same herbicide or same insecticide is used over and over, the nematodes that can survive on the PI88788 source of resistance multiply in the soil and the population numbers can increase.
There is a test to determine whether the nematodes in your field are resistant. However, the test is slow, expensive, may not tell the whole story, and generally is not recommended.
The easier way to monitor the effectiveness of your SCN-resistant soybeans is to test for SCN about every 6 years. If your first test was in the fall following a soybean crop, test 5 or 6 years later in the fall after soybeans.
If the numbers are holding steady or declining, your management plan is working. If the SCN numbers increase between tests, you may need to consider a longer rotation with non-host plants (alfalfa may be a good choice if it fits in your rotation) or you may need to look for a soybean variety with another source of resistance. However, these varieties make up less than 3% of all SCN-resistant varieties available.
Many soybean growers have asked about the effectiveness of seed treatments to reduce SCN numbers in the soil. To date, these products have shown varying results in their effectiveness against SCN. However, new products continue to come on the market and the University of Nebraska and other universities will continue to test their efficacy.
One thing should be noted about seed treatments: No seed treatment is intended to take the place of genetic resistance in combating SCN. They should only be used on resistant varieties for SCN-infested fields.
To see how SCN-resistant varieties performed in their ability to yield and effectively reduce SCN reproduction, see the Disease Management section under CW Soybeans to get results from the past couple years of trials. You also may want to review the results of similar trials conducted by Iowa State University.