Leaves are turning. You get the combine ready. In fact, you know that in some years, pods and beans can be dry even if stems are green. You don’t want to wait too long and risk losing overly dry soybeans to shattering at the header.

Finally, the day arrives when you believe it’s time to start combining. As you start down the first pass, you notice something odd. Scattered at random are a few single plants, each one green as grass. There aren’t many, but in a field of otherwise plants with no leaves, they are conspicuous.

I’ve encountered this scenario for three years in a soybean field that Farm Progress tracks all season to report observations. When it first appeared, careful inspection indicated that while the plant had leaves, pods were small and in clusters. Often, there was no seed inside pods. If there was, they were large beans, still at the green bean stage, with frost approaching. Those don’t make viable seeds.

That very first year, an agronomist suspected an unusual virus, tobacco ringspot virus. Samples were sent to the Purdue University Plant and Pest Diagnostic Lab. However, testing couldn’t confirm that it was TRSV. In fact, the lab staff couldn’t zero in on any single cause.

Rinse, lather and repeat! This process happened again in the next two years, with similar results spelled out in lab reports. But by the third year, growers in other counties were sending pictures of similar plants. There seemed to be no rhyme or reason — there were different seed brands and different maturities.

Related:Cause of green stem in soybeans eludes researchers

Possible answer?

A recent interview with Seth Naeve, Extension crop specialist at the University of Minnesota, provided insight. Discussing various suspected causes of green stem syndrome, he mentioned a special case: genetic mutation.

“These plants are likely due to a genetic mutation which produces clumps of small, underdeveloped pods,” he explains. In fact, they closely resemble male sterile plants incapable of producing viable seed. Naeve believes they are likely the result of a mutation that leads to male sterility.

“It’s often just single random plants scattered throughout a field, just like you described,” Naeve says. “This is distinctly different than typical green stem syndrome — where plants are mature and can yield well, but stems just stay greener at harvest, even when pods turn brown and beans dry down.”

Bingo! The light bulb flashed. Even though Naeve notes something else could have been occurring in these cases, things clicked. It seems like a strong possibility. But why do these plants stay so green?

“They appear capable of producing nutrient compounds through photosynthesis, but there is nowhere for nutrients to go in the plant,” Naeve explains. “Normally in later reproductive stages, compounds produced by the plant are moved from leaves and stems to beans inside pods. It’s called a ‘source and sink’ effect. Individual soybeans are the ‘sink,’ which receive materials transported from the ‘source,’ which are leaves and stems.

“We believe this may be the underlying explanation for why stems stay green in most or all cases of green stem syndrome, even if we don’t know for sure what causes it to happen. In this case, with virtually no beans, there is nowhere for nutrients to go. Plants just stay green, and at harvest, they stand out.”

They certainly do, and thanks for the explanation!