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An Integrated Approach to Enhance the Durability of SCN Resistance for Long-Term Strategic Management
Fri - August 26, 2016
SCN cysts. Photo: Iowa State University
SCN cysts. Photo: Iowa State University
by Brian Diers, Soybean Breeder, University of Illinois, and Thomas Baum, Soybean Nematologist,; Iowa State University

The soybean cyst nematode, Heterodera glycines, (SCN) is the most damaging pathogen of soybeans in North America. Although SCN-resistant soybean varieties are available to minimize yield loss, over 90% of the varieties available to growers in the north-central United States have the soybean line PI 88788 as the genetic source of resistance. The effectiveness of PI 88788 resistance has decreased over time as nematodes have adapted to this type of resistance. Producers are faced with limited options for rotation once virulent SCN populations develop in their fields.

Over the past 20 years, SCN resistance genes from other sources of resistance have been identified. However, these genes largely have not been transferred into soybean varieties. In a new project funded by the North Central Soybean Research Program, we are collaborating in a group of eight soybean plant breeders and nematologists in Illinois, Iowa and Missouri on research we hope will overcome the bottlenecks that have slowed the incorporation of these new genes into commercial varieties.

We began by evaluating over 200 experimental lines from the 2015 Northern Regional SCN Test and the Northern Uniform Test for the major SCN resistance locus known as Rhg1.  It has been shown that the number of gene copies at this locus impacts SCN resistance, with more copies conferring greater resistance. For example, Fayette, one of the first public varieties with PI 88788 resistance, has 10 copies at Rhg1.

We are finding that experimental lines differ in the number of gene copies at the Rhg1 locus, and have confirmed the correlation between the copy number and SCN resistance level.  We have been able to separate genotypes that have SCN resistance from PI 88788 from those with resistance from non-PI 88788 sources and from susceptible genotypes. This has provided breeders with a new tool to predict and maximize SCN resistance.

Seven genes from five soybean lines, including wild soybean (Glycine soja, have been identified that we are using to develop experimental lines with new combinations of resistance genes. The new lines are being tested for SCN resistance in SCN-infested environments and also tested in non-infested environments to determine if the genes and combination of genes affect yield. Gene combinations that are effective in controlling SCN, and are not associated with negative yield, will be bred into elite soybean germplasm.

In an initial test of the impact of SCN resistance on yield, experimental lines with five different resistance sources are being evaluated this summer in SCN-infested field locations in Illinois and Missouri. There are a total of 95 experimental lines that segregate for resistance genes from three resistance sources in these field tests.

To provide farmers with effective and durable rotation schemes, our next step is to test the selection pressure of gene combinations on SCN by growing rotations of the experimental lines in soil infested with soybean cyst nematode virulent on PI 88788 (HG type 2). The effect of rotation on the size of SCN populations will be measured after each generation for 12 generations. The goal is to identify gene combinations that when used in rotation will reduce the selection pressure on the SCN population, thereby slowing nematode adaptation to resistant cultivars.

We are also working to complete the SCN genome assembly (the genetic blueprint of the nematode) in order to identify the nematode genes required for the nematode to adapt to and reproduce on resistant cultivars. These types of genes are useful markers to monitor nematode population shifts in the field, and can provide plant breeders with another tool to identify new sources of resistance.

To read more about soybean cyst nematode, SCN-resistant varieties, rotation and other SCN management practices, please see
Soybean Cyst Nematode on this website
SCN Guide, 5th Edition North Central Soybean Research Program
Soybean Cyst Nematode Field Guide - Iowa State University
Soybean Cyst Nematode-Resistant Soybean Varieties for Iowa - Iowa State University