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Issue No. 23, Article 5/October 8, 2010

Soybean Plants and Stems Staying Green This Year?

Soybean harvest is on a fast track this year. The USDA-NASS crop report estimated 50% of the crop was harvested as of October 4, and with good weather forecasted through the rest of the week, the rapid pace will continue. In the last 10 years, only 2007 moved faster, with 54% harvested at a similar date. Early yield reports from growers have been mostly favorable, with numerous reports of field averages being the best some growers have ever seen. There seems to be little reason not to believe we will see the largest soybean yield average in history this year, as predicted by the last USDA yield estimate.


A green soybean plant in an otherwise mature soybean field near Decatur, Illinois. (Photo courtesy of Jeff Brown, agronomist with the Stone Seed Group.)


Green soybean plants in an otherwise mature soybean field near Morris, Illinois. (Photo courtesy of Russ Higgins, University of Illinois Extension IPM educator.)

One issue concerning some growers is green stems and green plants remaining in fields that are otherwise ready to harvest. I wrote about green, tough stems two weeks ago, but it was difficult at the time to predict whether they were a concern or just observations of the earlier-maturing soybeans. Since then, I've received several calls and pictures from agronomists and growers asking why so much green continues to appear when most of the field is ready to harvest.

Green stems and green plants can be somewhat separate issues, but from the combine they may be hard to sort out. Green stems, sometimes referred to as "green stem syndrome" or "green stem disorder," occur when stems remain green even though pods and seeds yield and mature fine (Egli and Bruening 2006). The condition can range from a nearly normal number of pods on a plant where the stems stay green to entire plants that remain green with few pods and no seeds developed.

In the latter case, entire plants that remain green can easily persist until a killing frost occurs. These situations can be entirely genetic in nature due to male sterility, in which case plants will set about 85% fewer pods, causing 4.5 times higher carbohydrate concentrations in the root, stem, and leaf matter (Wilson et al. 1978). Hill et al. (2006) evaluated 1,187 different MGI and MGII cultivars in Illinois from 2001 to 2004 and found some relationships between percentages of green stem and certain cultivars, suggesting better variety selection may be possible. However, the syndrome is illusive under different environments, and there is likely little information available to growers that can aid in their seed selection.

While genetics may play a role, these symptoms can also be almost entirely environmental (Egli and Bruening 2006). They are commonly associated with viral infections, primarily bean pod mottle virus and secondarily tobacco ringspot virus. They can also be caused by insects feeding on flowers. Stink bugs are a primary culprit, but bean leaf beetles and corn rootworm beetles are also suspects. Moreover, any other abiotic stress factors that increase flower abortion (causing pod loss), like drought, can play a role (Egli and Bruening 2006). With the number of potential causes for green stem syndrome, the culprit can be difficult to pinpoint when scouting at the end of the season.

But there is some good news. While it makes sense that individual less-productive plants reduce yield, green stem problems tend to appear in fields with average to high yields. One reason is that the green stems are a sign of favorable growing conditions throughout the maturity of the other plants. The only real concern for most growers is how much these green plants and stems reduce harvest speed. In most cases, the green plants constitute no more than 1% of the field. Harvest speed is not too affected at such levels when harvest conditions are dry.

In some severe cases, where green plants can be 10% or greater, harvest speed can certainly be reduced. Unfortunately, there are rarely clear answers between fields for why these symptoms appear, and little can be done even if the reason is evident. In cases with high percentages of green plants, delaying harvest until after a killing frost might be an option, but monitor the weather and the integrity of the other plants so you don't lose yield to lodging or shattering.--Vince M. Davis

References

Egli, D.B., and W.P. Bruening. 2006. "Depodding Causes Green-stem Syndrome in Soybean." Online. Crop Management doi:10.1094/CM-2006-0104-01-RS.

Hill, C.B., G.L. Hartman, R. Esgar, and H.A. Hobbs. 2006. "Field Evaluation of Green Stem Disorder in Soybean Cultivars." Crop Science 46:879-885.

Wilson, R.F., J.W. Burton, J.A. Buck, and C.A. Brim. 1978. "Studies on Genetic Male-Sterile Soybeans: Distribution of Plant Carbohydrate and Nitrogen During Development." Plant Physiology 61:838-841.

Author:
Vince Davis

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