Reports and questions have come in recently concerning individual soybean plants that are staying green in fields of brown soybean plants. It is natural to ask why this is happening. Frequently a first thought is to question whether the seed was contaminated or mixed with a different variety. Another common thought is that the variety may be expressing genetic mutations. It is unlikely that these are the reasons; the more likely explanation is that it is a problem called "green stem."|
Green stem is a problem in many areas of Illinois and in bordering states. It is not new but seems to have increased in the past 5 to 10 years. Green stem refers to plants that maintain green stems and leaves well past the time when they are normally brown and mature.
This problem is associated with yield loss due to reduced pod numbers and seed numbers per pod. Seed quality may also be affected because seeds from plants with green stem tend to express more mottled discoloration than seeds from healthy plants. Green stem can also cause delays in harvest and may disrupt harvest by obstructing combines.
Incidence of green stem varies from location to location in a given year and can vary from year to year. Differences for incidence of green stem among varieties and locations have been reported in Illinois this year. For example, in Monmouth, differences in incidence were observed among varieties. In addition, green stem incidence for one soybean variety varied from minimal to significant levels among three locations in central Illinois. In southern Wisconsin in recent years (http://www.plantpath.wisc.edu/soyhealth), incidence of green stem ranged from 1 to 50% in different fields. Similar observations have been reported for Illinois.
The cause of green stem is uncertain. This problem has been attributed to many different causes. These include plant viruses, low soil moisture, potassium deficiency, phytoplasmas, soybean population density, genetic mutations in soybean plants, and insect damage. Plant viruses seem to be most commonly associated with green stem. Bean pod mottle virus, which is transmitted by the bean leaf beetle, has been strongly associated with green stem. Soybean mosaic virus has also been linked to this disorder. However, specific viruses are not always detected in plants with green stem, and plants with specific viruses may not have green stem. Based on what is known at this time, green stem is probably caused by virus infection combined with other factors. Green stem and its causes are being studied at the University of Illinois and other midwestern universities. As more is learned, we will increase our ability to predict and manage this complex disorder of soybean.--Dean Malvick