50 Years of Soybean Cyst Nematode
|Terry L. Niblack
Department of Crop Sciences
Alison Colgrove, and Kamron Colgrove
The soybean cyst nematode (SCN) was first identified in southern Illinois nearly 50 years ago (Fig. 1). Since then, SCN has been confirmed in every county in our State (Fig. 2). In 2005, in order to improve our estimates of the costs of SCN to soybean producers, we surveyed soybean fields in Illinois to determine two things: 1) the actual distribution of the nematode and the probability that its numbers were high enough to reduce soybean yield; and 2) the SCN Types, which measure the ability of the nematode to attack resistant varieties.
Fig.1 Distribution of SCN in 1962 (source: USDA).
Fig. 2. Distribution of SCN in 2005 (source: R.D. Riggs)
We prepared for the survey by generating random GPS coordinates. To gather the data, we traveled around Illinois and took soil samples from the nearest soybean field to one of the previously determined coordinates. The samples were processed immediately for SCN egg counts, and then prepared for SCN Type testing in the greenhouse. We found that SCN infests 84% of the soybean fields in Illinois. The average number of SCN eggs was 2,700/100 cc soil, which is more than five times the measurable damage threshold of 500/100 cc soil. The range was 4 to 43,000 eggs, with a median of 2,100.
Fig. 3. Distribution of SCN Types in Illinois (preliminary data). Source of 1991 data: Sikora and Noel, 1991, J. Nematol. 23:624-628
As of this writing, the SCN Type tests are not complete, but the preliminary data are shocking. In 1991, most of the SCN populations in Illinois were SCN Type 0, equivalent to “race 3” in the old terminology. In practical terms, this meant that most SCN problems could be controlled by the use of SCN-resistant varieties. After 15 years of almost exclusive use of SCN-resistant varieties derived from the soybean line PI 88788, we have seen a complete shift in the SCN Type: most SCN populations (~80%) are now SCN Type 2 or 1.2, which means that they can attack PI 88788 and any resistant varieties derived from PI88788. This survey demonstrated how the use of SCN-resistant varieties can change SCN populations. Identification and deployment of novel sources of resistance to SCN should be pursued vigorously.