Farmers in some areas of the state are harvesting green "stemmy" soybeans this fall. The stems are tough and still green, although the pods are mature and dry and ready to be harvested. It's not all fields that have this "Green Stem Syndrome," but there are some. "We've been fielding questions and listening to some frustrated farmers," says Clarke McGrath, Iowa State University Extension field agronomist in western Iowa. McGrath writes the "Corn-Soybean Insight" column each month in Wallaces Farmer magazine. "Questions, concerns and frustrations are ramping up."
Harvesting green, stemmy 50-plus bushels per acre beans is frustrating. "Grinding through them with a $350,000 combine when they are yielding in the 20s and 30s is excruciating at best. Sort of like watching a KC Chiefs game only longer," he says.
Green Stem in Soybeans—symptoms, causes and bad news about management
In some situations McGrath has seen plants with mature pods on green stems. The petioles can be on or off, he's seen both of these symptoms in fields this year. He's also seen fields where the plants have few to no pods on the upper nodes. The leaves are still on the plants at the upper nodes in some spots in the field, and immature green pods are clustered on the upper plant in other places in the field.
Sometimes there's only a bean or two in the pods, sometimes there are empty pods, sometimes the pods are twisted and opened up.
Season-long soil moisture stress seems to be a trigger, especially after the plants have flowered, says McGrath. When a soybean plant is growing normally at the beginning of the season, it produces high levels of carbohydrates to feed itself; when drought or other stress hits, pods can fall off or not fill in relation to the carbohydrate pool in the plant. So, the plant has a pool of reserved carbs and may stay green longer. "That is our theory anyway," he says. "It is sort of like an otherwise healthy corn plant that doesn't make an ear for some reason -- the plant often turns purple from the anthocyanin levels."
Weather conditions for fast dry-down of grain, but not enough time for stems to dry
Lower fall humidity and higher fall temperatures can also contribute to this green soybean stem syndrome; you get conditions for fast dry-down for the grain, but not enough time for the stems to dry.
McGrath has read about the "green soybean stems at harvest" problem in the agronomy research literature. Studies suggest ALS herbicides can also contribute to the green soybean stem situation showing up, "but we haven't seen this herbicide interaction as a big factor in 2011 or so far in 2012," he says.
Fungicides can contribute to this green stem syndrome, too. "However, this year the drought limited the number of acres of fungicide applications so I don't expect this is a large factor this fall in causing the green stem situation we are seeing," says McGrath.
Also, this syndrome can happen with or without pressure from bean leaf beetle, soybean aphids, stinkbugs and thrips. But insect pressure does increase the prevalence of the related viral infections, since they are often vectors. What about spider mites? This insect is often a problem in doughty years, thus Iowa many Iowa soybean fields harbored infestations of spider mites during the summer of 2012.
Spider mite infestations likely contributed to increased green stem problem
"While I could find no data on spider mite damage and green stem syndrome, spider mites can contribute to earlier-than-normal pod shatter," says McGrath. "So in relation to overall plant condition, I believe spider mite infestations will generally leave us with green stems. Also, the yield hit our soybeans are taking from spider mites this year could contribute to carbohydrate accumulation in the soybean plant stems so it is possible that spider mites are a contributing factor"
Some researchers say green stem syndrome may happen if certain viral diseases are present in the field. "There is no documented direct correlation between green stem syndrome and these diseases, to my knowledge," says McGrath. "But yes, these diseases make the issue worse if present and they would add to the acreage reported to have the symptoms, since the diseases and the symptoms of green stem syndrome tend to mimic each other. A lot of plant pathologists think there is a separate viral or fungal pathogen that causes green stem syndrome, yet to be identified. Some do not think there is a viral cause -- they believe green stem has a physiological cause."
Does soybean variety make a difference in getting green stem syndrome?
"If you believe what the plant pathologists from Argentina say, they tell us they can't isolate a viral cause at all," says McGrath. "They have done some preliminary work from what I understand. Studies conducted in Japan show that pod loss is a direct cause of green stem. That relates back to the carbohydrate reserve theory."
Some people talk about how certain varieties of soybeans tend to have more problems with green stem syndrome showing up than do other soybean varieties, but this is really hard to track down, says McGrath. "Most of what is being said about soybean varieties and green stem syndrome is observation information or what the seed dealers are sharing anecdotally. I don't see farmers being able to make any soybean variety decisions based on whether or not we think one bean variety is more prone to have green stem syndrome than another variety."
What can you do about green stem syndrome and harvesting those beans?
"Not much," says McGrath. "There has been some talk of spraying Gramoxone to desiccate the plants, knock the leaves off, and speed the process up. When I worked as a retail agronomist in an ag chemical dealership, we tried spraying Gramoxone on soybeans with no tangible success. Maybe other people have had different experiences, so feel free to share them with me."
Grinding through the fields with your combine trying to harvest these green beans prior to a hard frost is likely the best option you can take. However, farmers point out that waiting for stems to dry is risky as pods can split or shatter, beans can become too dry, or bad weather can wreck fields. "The drought our beans went through in Iowa also appears to have created increased issues with early pod splitting in many fields, so waiting for stems to dry is extra risky this year," says McGrath.