Once a farm is known to be infested with Soybean Cyst Nematode (SCN), (i.e. eggs 1 to 10,000 eggs/ 100cc soil), proper crop rotation and soybean variety selection will be needed to manage SCN. 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 MN 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 a 2002 survey, Race 3 was the most common race in Minnesota, representing 77.6% of the sampled populations (Zheng et al., 2006). The remaining populations were composed of Race 1 (15.3%), Race 6 (3.5%), Race 5 (2.4%), and Race 9 (1.2%). Race composition can vary widely from the Southern U.S. to the Northern U.S. making it very important to use variety trial results from Minnesota when selecting resistant varieties.

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 20th. The average SCN egg count at the time of planting was 5,700 eggs/ 100 cc of soil. The trial tested a number of the commercially available SCN resistant and susceptible varieties (Table 1). 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 CystX® resistance. Soybean varieties containing CystX® have been reported to yield better than most resistant varieties when SCN egg counts are greater than 10,000 eggs/cc of soil, a situation where it is typically not recommended to plant resistant or susceptible soybeans. The CystX® varieties, however, had the lowest RI.

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. In Minnesota most commercially available resistant varieties utilize the PI 88788 resistant source and a smaller number derive their resistance from the Peking and CystX (PI 4376754) resistance sources.

Table 1. 2006 Soybean Cyst Nematode Resistant and Susceptible Soybean Variety Trial, Kasson, MN.
₁ RM=Relative Maturity, relative maturity and type information were supplied by the seed companies
₂ Egg=eggs per 100 cc of soil after R6
₃ RI=Reproductive Index ([eggs per 100 cc of soil after R6] /[5,700 eggs/100 cc of soil])
₄ Yield= bushels per acre adjusted to 13% moisture

For further information on resistant varieties:

When selecting varieties it is important to make selections based on data from as many site years as possible. The data represented in Table 1 are from one site year only, but are presented as they illustrate the importance of selecting the correct variety for managing SCN. For further University of MN SCN trial results, check out the following website: www.soybeans.umn.edu/crop/variety/State_Trials/index.htm

For further information on management recommendations:

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