Value of Alternating SCN Sources of Resistance

INSIGHTS

• Sustainable management of SCN requires the use multiple tools.

• Alternating soybean sources of resistance to SCN is an important tool for season-long protection.

Figure 1. Soybean cyst nematode and eggs.

Soybean cyst nematode (SCN) is the most damaging soybean pathogen in North America (Figure 1). Under severe SCN pressure with no cyst protection farmers can experience up to 50% yield loss, and even fields with no visual symptoms can see up to a 10% yield loss.¹

Once SCN is introduced into a field, it can never be eradicated. Because of that, it is a pest that must be managed; otherwise, it will eventually become a significant problem. Losses associated with SCN in any given year will be directly dependent on environmental factors, such as drought or other natural events. However, through planning and use of SCN management strategies, such as resistant soybean varieties, the impact of these SCN-related losses can be reduced.

Protecting Your Yield Potential

While using seed treatment products can provide a second mode of action to deliver SCN suppression for your soybeans, genetic resistance to SCN is the most reliable measure against this pathogen, as it provides longer window of protection.

For more than 20 years, greater than 95% of all SCN-resistant soybean varieties have utilized genes from the PI 88788 breeding lines as the primary source of resistance.¹ With the continued use of PI 88788, even in a rotation with a non-host crop, a SCN “race shift” can be experienced and increased cyst reproduction rates can be seen on varieties using this source of resistance. The term “race shift” is terminology used to differentiate and describe SCN populations based on their ability to reproduce on a specified set of soybean genetic cultivars or “indicator lines”.

What this means is that PI 88788 historically reduced the number of eggs produced by SCN to less than 10% of what was produced on susceptible varieties. In recent years it is not uncommon to see reproduction rates significantly higher than 10% on PI88788 sources of resistance, potentially resulting in higher yield losses.¹ That doesn’t mean that PI 88788 is not a valuable tool to manage SCN. When rotated with other sources of resistance such as Peking, it can offer more years of protection.

Newer resistant varieties of soybeans have recently been adapted for areas of the U.S. More genetic resistance options for growers means more options for SCN management.

What Does “Race Shift” Look Like?

Where PI 88788 effectiveness at managing SCN populations has been reduced, varieties with an alternative source of resistance like Peking can be a great tool to help reduce SCN populations and potential yield loss. Pictured is an example of a specific field where race shift has reduced the effectiveness of PI88788 varieties (Figure 2). The two Peking varieties show minimal impact, but the PI 88788 varieties exhibit severe stunting. Additionally, even in the absence of visual symptoms yield loss may still be occurring.²

Figure 2. Soybean trial evaluating soybean genetic resistance to SCN.

In 2018 yield trials within the same field where race shifts occurred, Peking varieties yielded 10-20 bushels more than PI 88788 varieties (Syngenta R&D 2018). Although uncommon to see such extreme visual symptoms and yield loss as in this field, it may foreshadow what could be an everyday occurrence if diversified management practices are not implemented more broadly.

The Peking source of SCN resistance has a different mechanism for SCN resistance than PI88788. Thus, rotation of SCN sources of resistance (PI88788 and Peking) is strongly encouraged to slow the development of SCN resistance and limit SCN reproduction and economic injury levels. Additionally, limiting SCN reproduction will reduce root damage caused by nematodes that can open the door for late-season diseases like Sudden Death Syndrome (SDS) and Brown Stem Rot (BSR) which can further reduce yield. Overall, it is important to

1. Test fields to know your SCN numbers
2. Rotate varieties with alternate sources of resistance
3. Rotate to non-host crops and
4. Consider using nematode protectant seed treatments