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Terry L. Niblack
Professor of Soybean Cyst
Department of Crop Sciences
Most SCN-Resistant Soybean Varieties Are NOT Resistant to SCN. 2005 Update on SCN Races in Illinois.
Yes, you read the title right: no matter what the seed label says, most soybean cyst nematode (SCN)-resistant soybean varieties are NOT resistant to SCN. At least, not to the SCN populations we have in Illinois.
How can this be? The nematodes have changed.
The last time there was a good survey of SCN races1 in Illinois was in 1989-902. Back then, 88 of 102 counties contained one or more fields known to be infested with SCN. The results of the survey showed that 64% of the SCN populations in Illinois were classified as race 3. That meant, basically, that nearly two-thirds of the SCN in Illinois could not attack any known resistant varieties.
What about the one-third that could attack resistant varieties? These were classified as races 1, 2, 4, and 5. And although this sounds like many SCN populations were able to attack and damage resistant varieties, actually most of them would be classified as "weak" today. In fact, only one SCN population (from Grundy County) was a "strong" race 1 that could be expected to damage race 3-resistant soybean.
What has happened in the last 15 years?
- Now, we know that there are SCN infestations in 100% of the counties in Illinois.
- In fact, more than 80% of the soybean fields in Illinois are infested with SCN.
- Our latest survey of SCN populations in Illinois shows that 74% can attack most resistant varieties.
But, just because these SCN populations can attack resistant varieties doesn't necessarily mean they can reduce soybean yield. That depends on three other factors:
How can you make sure the varieties you're using ARE resistant to SCN? The following recommendations correspond to the three factors listed in the paragraph above:
- how many SCN are present (usually an egg count is the best way to tell, especially if resistant varieties have been grown in the field)
- whether SCN-resistant varieties have been grown in the field in the past and were rotated properly; and
- how resistant the variety is (the one that's being grown in the field now).
- Sample your SCN-infested fields at least once every 4 years and keep track of the numbers. If they go up, you need to track down the problem. We can test your SCN population to help you make variety selections, if necessary.
- Never use the same SCN-resistant variety in the same field twice in a row - even if you grow corn for a year in between. It only takes 5 or 6 generations for SCN to adapt to a variety, and 2 seasons gives it plenty of time.
- Always check the Variety Information Program for Soybeans (VIPS) database online (at http://www.vipsoybeans.org/) or contact me for a printed version (firstname.lastname@example.org) to see what the actual level of resistance is in your "SCN-resistant" variety. There's a lot of variation in the level of resistance in commercial varieties labeled as resistant to SCN (Fig. 1).
Figure 1: In 2004, over 630 SCN-resistant varieties were tested for their levels of resistance to an SCN population representing a common type in Illinois (type 2, or old race 1). We found that most varieties were not resistant to this type, and therefore not resistant to most SCN populations in Illinois. HR = highly resistant; R = resistant; MR = moderately resistant; LR = low resistance; and NR = no effective resistance.
1As you know, we don't talk about races in Illinois anymore. For further information on why, see Niblack et al., 2002. A revised classification scheme for genetically diverse populations of Heterodera glycines. Journal of Nematology 34:279-288. Available online at http://palmm.fcla.edu/nematode/.
2 For further information, see Sikora, E., and Noel, G. R. 1991. Distribution of Heterodera glycines races in Illinois. Supplement to Journal of Nematology 23:624-628. Available online at http://palmm.fcla.edu/nematode/.