Green stem disorder has been observed in many fields throughout Delaware and Maryland this season. This disorder is characterized by plants that remain green in parts of a field, thereby preventing a timely harvest. Sometimes green stem occurs in single plants, other times large areas of the field can be affected.
It is not known what causes green stem disorder, although research has shown that factors that reduce pod or seed number can result in green stem. This is likely because the foliage and stems retain the capacity to produce carbohydrates, and without adequate pods or grains, there is no sink for these carbohydrates later in the growing season. Excess carbohydrates are retained in stem and foliage, resulting in plants that stay greener later in the season- at least that is the working hypothesis.
There have been several factors associated with green stem, although none has been shown to cause green stem in a consistent fashion. These include environmental issues such as compaction, water stress, nutrient (potassium in particular) and pH issues, insect damage, and viruses. Typically these factors limit pod or grain number at the end of the season, thereby resulting in green stem symptoms. Remember, association is not necessarily indicative of causation. Just because something is associated with something does not mean it is the cause. For example, let’s say a neighborhood that has the most streetlights also has the greatest levels of crime. Does that mean more lights means more crime? It sounds a bit silly, but that is what we need to keep in mind here-we have a little of association with little to no proof of causation. Even then, the association is weak.
In the past some viruses were considered to be a major factor in greenstem disorders; however, several research studies, for example- Hobbs, H.A., et al., Green Stem Disorder of Soybean. Plant Disease, 2006. 90(4): p. 513-518, have shown that there is no relationship between viruses and green stem- plants may have green stem and not have virus and vice versa. There could be, of course, some unidentified virus that causes the disorder, or it could be a combination of factors. The bottom line is nobody knows for sure and anyone that says they do…..well….
As far as management is concerned, follow good management practices and ensure that plants are properly fertilized and planted at optimal, but not excessive, populations. Ensure that plants are receiving adequate water after pod fill if fields are irrigated and ensure that fields are scouted regularly and managed for diseases and insects throughout the season. Cultivar selection also can be important as some cultivars seem to be more or less tolerant to the disorder than others.