While many soybean fields are drying normally and harvest is underway, University of Illinois crop sciences professor Emerson Nafziger has heard reports of soybean plants with green leaves and green stems that have dry pods and seeds.
Many of these fields have too much green tissue to combine, but are in danger of shattering as pods and seeds continue to dry. Nafziger thinks this may be the "green stem syndrome" that has cropped up in some fields in recent years.
"We think that this occurs when seeds stop taking up sugars and nitrogen before senescence is complete," he notes. "When this happens, stems and petioles continue to photosynthesize, but sugars have no place to go, so they accumulate and keep leaf and stem tissues healthy and green. Lower temperatures and more soil moisture recently have likely contributed to this problem by keeping leaves well-supplied with water and slowing sugar movement and leaf-drying rates."
Although the syndrome is known, its causes are not well understood. One well-known cause is lack of pods. When disease, drought, or insect damage results in low pod numbers, either the seeds and pods do not send the normal senescence signal, or it is not received normally. Thus, the leaves do not lose their color and drop off the plant. Rather, they stay green and attached and continue to produce sugars.
"We think that lack of pods in drought-stressed fields is one of the factors leading to this problem in 2012, but there are two unusual features this year," Nafziger explains. "One is that pods seem to be maturing and drying normally. In many previous cases, pods tended to stay green or dry slowly when pod numbers were low."
Nafziger also notes that in some fields this year, only the lower leaves have stayed green and attached to the stem, while the upper leaves senesced normally. Both of these symptoms may be related to earlier drought conditions.