Issue No. 22, Article 2/September 7, 2007
Distribution and Virulence of the Soybean Cyst Nematode in Illinois
We conducted a survey of soybean fields in Illinois during 2005, sampling randomly selected GPS coordinates along state and county highways (see Figure 1). We arbitrarily divided the state into six geographical sections (southeast, southwest, east central, west central, northeast, and northwest) to allow us to collect all the samples from a particular section on a single day. We found that 83% of fields are infested with soybean cyst nematode (SCN), with average population densities high enough to cause significant yield suppression (see Table 1). The "damage threshold" for Illinois is 500 eggs per 100 cc soil.
Figure 1. Map of Illinois showing the locations of survey sample sites in six arbitrarily designated geographic sections (differentiated by contiguous dots of the same color). Soil samples were collected at these locations for analysis of soybean cyst nematode populations in 2005.
A preliminary study, conducted in 2003 in collaboration with Jason Bond at Southern Illinois University (and reported in the Bulletin in 2004), showed that 82% of the SCN populations in southern Illinois were able to attack Plant Introduction (PI) 88788, the source of resistance in more than 90% of the SCN-resistant varieties grown in the state. The adaptation of SCN to PI 88788 limits the effectiveness of this resistance in protecting soybean yields and reducing SCN populations. To determine whether this adaptation has occurred elsewhere, we selected 156 of the 218 samples collected in the survey to test their adaptation to resistant varieties (Table 2) during 2006 and 2007 by means of the Illinois SCN Type test.
For those unfamiliar with the SCN Type test: the Illinois SCN Type is determined by the ability of the nematode population to develop on three indicator lines: PI 548402, PI 88788, and PI 437654. A Type 0 cannot develop on any of the three; a Type 1 develops on PI 548402, a Type 2 on PI 88788, and a Type 4 on PI 437654. (There is no Type 3 in this test, although there is in the full HG Type test). A Type 1.2 develops on both PI 548402 and 88788. Development is considered positive when the female index (FI, defined in footnote c of Table 2) is 10 or higher.
To show how SCN populations have changed over time, we compared our results to those reported by Sikora and Noel (1991; see Figure 2). At that time, SCN adaptation was measured by means of a race test. Sikora and Noel's interest was in the percentage of SCN populations that could be described as "race 3," defined as those without the ability to develop on any of the four differentials--in other words, those that could be managed through the use of any SCN-resistant cultivar. A graphical comparison of the results of the 1991 survey and the results of our 2005 survey showed a distinct virulence shift (what used to be known as a "race shift"). This shift has most likely been due to the fact that most of the SCN-resistant cultivars available in Illinois derive their resistance from PI 88788, and most Illinois soybean farmers are choosing to plant SCN-resistant cultivars.
Figure 2. Comparison of the results of surveys of soybean cyst nematode populations in Illinois. The "1991" data were reported by Sikora and Noel as "races" and were expressed here as Illinois SCN Types in order to make the comparison clearer.
These results show clearly that most of the SCN populations in Illinois have adapted to some extent to PI 88788. Very few populations were able to develop on PI 548402 (SCN Types 1 and 1.2), and none were found that could develop on PI 437654 (SCN Type 4) during this survey (although 30% of the populations were able to produce one or more viable females on this PI; Colgrove and Niblack, unpublished data). Cultivars with resistance derived from PI 437654 (such as CystX) are not yet deployed widely in Illinois.
Despite the common use of nonhost rotation and host resistance to reduce the impact of SCN, its population densities remain at levels capable of causing significant yield loss in Illinois. The lack of obvious symptoms reduces recognition of the extent of SCN infestations, and the adaptation of SCN populations to most resistant cultivars reduces their effectiveness in alleviating damage. However, this survey showed that the nematode has not often adapted to PI 548402 or 437654 in Illinois, so the option to use these sources of resistance remains open and should be encouraged, especially when SCN population densities are high. The use of alternative sources of resistance will benefit farmers with heavy infestations of SCN Type 2 in the short term, and widespread rotation of sources of resistance should benefit the soybean production industry in the long term by slowing the nematodes' adaptation to resistance and preserving the effectiveness of alternative sources.
This research was supported by the Illinois Soybean Association, the Soybean Disease Biotechnology Center, and the North Central Soybean Research Program. Thanks to M. Greifenkamp for preparing Figure 1.
Reference: Sikora, E.J., and Noel, G.R. 1991. Distribution of Heterodera glycines races in Illinois. Journal of Nematology 23(4S):624-628.