Issue No. 15, Article 4/July 6, 2007
The Case of the Yellow Beans
In the past couple of weeks, many people have noticed medium to large patches of stunted, chlorotic soybean plants in central Illinois. A limited survey of 32 similar patches in Champaign and Piatt counties last week revealed the following:
- only about half of the patches were associated with obvious "low spots" in the field;
- 100% of the plants dug up in the patches showed evidence of soybean cyst nematode (SCN) infestation, but so did the nonsymptomatic plants outside the patches;
- 100% of the plants observed showed evidence of severe root growth restriction and root rotting, compared with their nonsymptomatic cohorts outside the patches;
- unlike in previous years, there were infrequent diagnostic signs or symptoms of Phytophthora, Pythium, or Rhizoctonia on the plants inside or outside of the patches;
- there is no way of knowing whether the fields exhibiting symptoms are those planted to SCN-susceptible varieties (more than half of soybean fields in Illinois are planted to SCN-resistant varieties).
Champaign County, July 2, 2007.
The question being asked frequently is, "Are these patches caused by SCN?" The answer is a dissatisfying, "Could be " These symptoms are classic for SCN in the south-eastern states, but we do not usually see them so clearly in the Midwest. Last year, we predicted a much larger than normal SCN egg population density for this year (on average), but huge numbers of SCN by themselves are not always associated with obvious symptoms. Did the symptoms show up because root damage was exacerbated by the long rainless period we had? This last idea is a likely explanation. We know that SCN damage (yield reduction) is more pronounced when water is limiting. The rotting visible on heavily SCN-infested roots was probably due to opportunistic, root-rotting organisms taking advantage of the weakened roots and encouraged by the rain many of the parched areas had in the third week of June.
The Case of the Yellow Beans is still unsolved. (Unfortunately, the symptoms did not show up in any heavily SCN-infested research plots.) However, SCN is likely to have been involved in the development of many of these patches, and the plants will not recover in terms of yield, even if the symptoms abate.--Terry Niblack