The Resistance In All SCN-Resistant Varieties Is Not the Same: The Untold Story

	Brian Diers Associate Professor Department of Crop Sciences (217) 265-4062; bdiers@illinois.edu
		Greg Noel USDA-ARS and Professor Department of Crop Sciences (217) 244-3254; g-noel1@illinois.edu

Soybean varieties sold in Illinois are typically labeled as resistant or susceptible to soybean cyst nematode (SCN) race 3 or 14. Unfortunately, this labeling does not consider the different levels of resistance to SCN. In a recent screening of varieties labeled as SCN resistant, University of Illinois researchers found that resistance levels of these varieties varied from resistant to susceptible (Figure 1).

Figure 1. Frequency distribution of the reaction of 300 proprietary SCN-resistant varieties
to SCN race 3 (HgType 0). The Female Index is the percentage of SCN
reproduction on the variety compared to the susceptible control (100%).
R = resistant, MR = moderately resistant, MS = moderately susceptible, and S = susceptible.

There are several reasons for this variability in resistance. The first is that unlike resistance to some soybean disease such as phytophthora rot, resistance to SCN is controlled by many genes. For example, SCN resistance in PI 88788, the most common source of SCN resistance in soybean varieties in Illinois, is controlled by at least three genes. These three genes must be incorporated into a soybean variety for full resistance to occur. This incorporation is difficult because in breeding populations, few experimental lines will carry all of these genes. It also is difficult to obtain the good SCN resistance scores that are needed to identify those resistant lines. During breeding, there also can be selection against SCN-resistant lines because these lines often are not the highest yielding. This negative association between resistance and yield occurs because resistance genes may be linked to other genes that reduce yield.

In addition to complexity in the inheritance of resistance, the nematode also is diverse in its ability to infect plants. When researchers test for resistance, a soybean variety may be rated as resistant or susceptible depending on the population of nematode used in the test. In any given field, the population of SCN may contain individuals with a wide range in ability to infect resistant varieties. This diversity will be better described in the new SCN classification system (Hg type) than in the race system that is in use at present.

When purchasing soybean varieties, consider the following:

Do not be fooled by a lack of symptoms in the field. SCN can cause a 10 to 15 percent loss in yield with no apparent symptoms.
A yield monitor is very helpful in determining where SCN infestations might occur. Take soil samples in areas that have a lower yield.
A soil sample that contains no cysts does not mean SCN is not present in the field from which the sample was taken. In a field that has 1 million cysts per acre, there is only a 50 percent chance of finding a cyst.
If an SCN infestation is suspected but not proven, select a high-yielding, SCN-resistant variety from the variety testing program.
In fields with a known history of SCN, the nematode will never go away. Continue rotating and plant a variety with a high level of resistance.

We hope to have SCN resistance ratings available on the VIPS/Stratsoy web page and in printed media late this fall.

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Participating Departments
College of Agriculture, Consumer and Environmental Sciences
University of Illinois Extension