Start with a good sample.

Although soybean roots can be examined for soybean cyst nematode females during the growing season, the best estimate of SCN densities is achieved through a good soil sample.

After the crop is off and prior to field work is an ideal time to sample fields slated for soybeans in the upcoming growing season.

A few key points about soil sampling:

• A soil probe is the ideal sampling tool although a trowel can be used
• Limit area sampled to approximately 20 acres
• Samples should consist of about 20 cores taken in a zig zag pattern throughout the field, to a depth of 6 to 8 inches
• Combine and mix samples thoroughly, placing 1 pt of soil into a soil sample or plastic bag
• Properly label sample bag
• If sample cannot be sent immediately, store in refrigerator
• Do not expose samples to direct sunlight for an extended time

After the sampling

Once a farm is known to be infested with SCN, proper crop rotation and soybean variety selection will be needed to manage the pest. When a field is infested with SCN, factors other than yield, such as level of SCN resistance and the source of resistance, should be considered.

Resistance to SCN is not an “all or nothing” trait because there are many genes involved in creating resistance and not all resistant varieties will have the same genes. The University of Minnesota rates soybeans as susceptible, moderately susceptible, moderately resistant, or resistant to SCN. Varieties in field tests are rated for SCN resistance based on the reproductive index (RI) which is calculated by dividing the egg count after R6 by the egg count at planting. Varieties with a reproductive index of 0.5 or less are generally considered resistant.

To further complicate SCN variety selection, there are a number of SCN races or HG types that have been identified. Soybean varieties vary not only in their resistance to SCN but in their resistance to specific races, which stems from the breeding process and sources of SCN resistance.

In 2006 a SCN variety trial was conducted in Kasson, Minnesota, at a site known to be infested with SCN. The study was a randomized complete block design with four replications, and each variety was seeded at a rate of 138,000 seeds per acre. Seeds were planted at 1.5 inches deep in 30 inch rows on May 20. The average SCN egg count at the time of planting was 5,700 eggs per 100 cubic centimeters of soil. The trial tested a number of the commercially available SCN resistant and susceptible varieties. The selected varieties were evaluated for yield and impact on SCN egg counts.

At this site, resistant varieties tended to yield better than the susceptible varieties with the exception of the soybeans containing resistance. Resistant soybean varieties have been reported to yield better than most resistant varieties when SCN egg counts are greater than 10,000 eggs per cc of soil, a situation where it is typically not recommended to plant resistant or susceptible soybeans. The resistant varieties, however, had the lowest reproductive index.

As expected, the susceptible varieties yielded less and caused a substantial increase in the number of SCN eggs. One important note is that at the current time all resistant varieties allow some SCN reproduction regardless of the source of resistance, and no resistant soybean variety will eliminate SCN from an infested farm. Because reproduction is possible on all resistant varieties, the University of Minnesota recommends rotating to varieties with different sources of resistance and at the very least rotating to different resistant varieties.