What causes green bean syndrome?

Some areas of soybean fields are not harvested each year due to what is known as the green bean effect or green bean syndrome. This phenomenon is not widely understood, but it is attributed to a combination of factors, with stinkbugs and aerial blight being the most common causes.

Across Louisiana we have seen heavy infestations of stinkbugs and aerial blight reported farther north than usual. Some producers already have not harvested some parts of Group IV fields because of the green bean syndrome.

A developing soybean pod is sensitive to punctures or piercing. Stinkbugs are notorious for “hitting” pods and causing seed and pod abortion, which can reduce yield.

Aerial blight infestations which cause heavy pod drop during the growing season can cause plants to reflower for two to three weeks while the plant tries to reset pods.

This leads to different maturity stages within the same field, which can be diagnosed as green bean syndrome. However, green bean syndrome is usually associated with plants that seem to never mature.

According to Matt Baur, LSU entomologist, “There doesn't seem to be one causal agent that actually triggers green bean syndrome… rather a complex of agents acting in similar fashion such as aerial blight, stinkbugs or other insects such as the leaf hopper. Research is limited as to why this occurs but somewhat more definitive regarding some of the causal agents.

“LSU researchers have looked at stinkbugs and aerial blight causing pod loss, which may not be the specific cause of green bean syndrome, but there is a correlation between them. There is evidence suggesting that the leaf hopper may transmit a mycoplasma, which could also affect the maturing process in soybeans.”

Jack Baldwin, LSU Extension entomologist, says, “Stinkbugs and aerial blight are probably the main reasons soybeans mature at different times during the growing season. The reasoning as to why this occurs is not fully understood and is very difficult to assess because of the different insect and environmental conditions that occur yearly.”

Ken Whitam, LSU Extension pathologist, adds: “Yield loss estimates for green bean syndrome are difficult to assess because other diseases play roles in yield loss.”

From a physiological standpoint, what is going on in the plant is not fully understood, but Jim Board, LSU soybean physiologist, says, “The plant produces growth regulators that it sends to parts of the plant to signal maturity. Research is demonstrating that when the plant is in the R5 stage it plant will release an auxin, probably (IAA) from the young developing seed. This auxin signals the plant to stop branching and continuation of vegetative growth and concentrate on seed filling.

“If nothing has hindered this process, the plant matures naturally. If something does interfere with this process, the soybeans will continue to branch and grow vegetatively for as long as possible, causing different maturity stages in the field. In the field, anything (such as stinkbugs or diseases) that interrupts the seed sending the auxin to other parts of the plants will inhibit natural maturity.”

There is no way of determining to what extent green bean syndrome will affect this year's crop, but we suspect that in some cases where stinkbugs were allowed to harbor a little too long or where we were a little late spraying for aerial blight, it has the potential to show up.

There is some consistency to the green bean syndrome. In a heavy stinkbug and aerial blight year (like this year), green bean syndrome tends to occur more frequently.

David Lanclos is the Extension soybean specialist with the LSU AgCenter. He is located at the Dean Lee Research Station near Alexandria, La.

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