Seeing green — stems, not cash
A text message I received in mid-October: “Beans look great, 11% moisture, 60-plus bushels per acre. But 90% green stems so harvesting at 2-3 mph. What the heck? Anyone know why these things are so stemmy?”
This is a common question agronomists received this fall: What is going on with these green, chewy soybean stems? In visiting fields and doing plot work near some of these bean fields, I could hear the combines shuddering and growling to chew through these acres. Aptly named green stem disorder, or GSD, we’ve fielded questions and reports on this for years. But for the last four or five years, it seems to have gotten progressively worse.
This year GSD was scattered across much of the state again, and the late harvest season coupled with uncooperative weather in early October really had farmers frustrated. They were fed up with trying to grind through some of these bean fields, and were looking for ways to avoid having to deal with GSD in the future.
What is green stem disorder?
Green stem disorder is loosely defined as a soybean disorder in which stems remain green after pods and seeds are fully mature and ready to harvest. Cutting affected plants during harvest is difficult, and seed quality can be reduced. I’ve seen machines where the stems wrapped around the reel like fishing line on a cheap rod after a bad cast; and if you aren’t careful, the stemmy mess can slug the combine’s throat.
We see all sorts of plant symptoms in these fields in addition to green stems, so a textbook description is hard to give. Sometimes, plants with GSD can have just a few pods on the upper nodes, or there can be leaves still on at upper nodes in some areas of the field. Also, immature green pods may be clustered on the upper part of the plant, and empty pods have been associated with GSD.
Bottom line: if there are any mature pods in a field ready to harvest, and green, chewy stems are also present, this is considered GSD by our definition. Looking at it that way, there may have been even more GSD out there than originally thought; it’s just the most obstinate fields that get most of our attention.
Depending on location, many different factors have been thought to be associated with GSD, including viruses (especially beanpod mottle virus), soybean aphids, thrips, stinkbugs, leafhoppers, bean leaf beetles and cercospora leaf blight. However, USDA research in Illinois showed no relationship between green stem disorder and many of these factors. This doesn’t necessarily mean these factors don’t cause green stem disorder, but what is evident is you can get GSD without these factors. Also, some of the symptoms described in scientific literature are not what we see in Iowa fields.
Some plant pathologists think there is a separate viral or fungal pathogen that causes GSD, and it is yet to be identified. Some think it is physiological. Evidence for a physiological effect comes from a Kentucky research project where they dramatically increased GSD by severe pod removal.
What conditions can cause a reduction in pods and an increase in GSD? Over the past few years, season-long soil moisture stress (both too little and too much) seems to be a trigger, especially post-flowering. When a soybean plant is growing normally at the beginning of the season, it produces high levels of carbohydrates to feed itself. When stress hits, pods can fall off or not fill in relation to the carbohydrate supply in the plant. If the plant has a pool of carbohydrates in reserve, it may stay green longer.
Pod removal research in Japan (where GSD has been a serious challenge for their soybean growers) showed that partially depodded plants had higher carbohydrate supplies in the root and stems. These plants, with fewer pods to deposit their reserves into, matured later than fully podded plants. The partially depodded plants had higher water absorption and higher stem moisture at pod maturity. In other words, they had GSD. This seems to add credence to the Kentucky work.
Soybean variety a factor?
Other factors associated with GSD, at least anecdotally, include lower fall humidity and higher fall temperatures that lead to faster drydown for the grain and not enough time for stems to dry. Plant populations also have been associated with GSD, but oddly, both lower and higher populations have been blamed as the problem.
Varietal tendencies are hard to accurately assess. Some agronomists advise the best thing to do is to note varieties that have more GSD and avoid them. On the other hand, I’ve experienced that often GSD seems to be driven more by environmental factors than by genetics.
If you’ve had consistent GSD issues with a variety, then that is solid advice; otherwise, if a variety you liked performed well in several fields and only had GSD in certain areas, it’s hard to toss it aside when booking seed for 2015.
McGrath is the ISU Extension field agronomist at Harlan in western Iowa.
This article published in the November, 2014 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2014.