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."