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What is causing yellow flash in soybeans?

Posted Wednesday, August 17, 2011, at 8:59 AM

The yellow flash that we have seen in soybeans this year seems to be lasting longer than usual. We usually see some of this yellow where there is an overlap or a particularly high rate of glyphosate used.

Stressed beans will flash more readily and with the heat and dry weather stress that we have experienced many fields have flashed even with typical rates of glyphosate usage.

According to several sources, yellow flash is the temporary chlorosis of newly emerging soybean leaves that sometimes occurs following the application of glyphosate to glyphosate-resistant soybeans.

Yellow flash is most likely to occur when environmental conditions are warm and moist, favoring rapid soybean growth, and/ or with high rates of glyphosate are applied. The temporary chlorosis results from a reduction in chlorophyll content in the affected leaves.

A few days after glyphosate is applied, all leaves on the soybean plant remain green except the newest leaves at the top that were less than 1/2 inch long at the time of the spraying.

These leaves continue to grow and expand, but the chlorophyll production is reduced, leaving a yellow color that lasts normally up to a week. In the case of yellow flash, glyphosate does not cause green leaves to turn yellow; instead, temporary yellowing is a result of leaf development when the soybean plant is under multiple stresses.

If soybeans are under continual stresses, yellow leaves may appear until 10 to 21 days later. Under dry conditions, yellow leaves may stay yellow until the crop resumes growth after rain releases the crop form stress. Yellow flash is largely a result of an interaction of the with the environment and not the genotype of the plant.

It is important to make sure that the yellowing is not a result of a nutrient deficiency. Nutrient deficiencies such as iron chlorosis, and manganese and possibly other nutrients could produce similar symptoms to yellow flash.

However, these nutrient deficiencies would be expected earlier in the season and soybeans would have grown out of these conditions by now.

Yellow flash by itself has been reported as having no effect on yield, but any associated stress factors may affect yield to a degree.

Stress can delay pod set and with a shorter period of time for pod fill, yields may be reduced some.

Pod set delay may also reduce the number of pods compared to more optimal growing conditions.

Another yield component that may be affected is the number of beans per pod. However, soybeans are a very resilient plant with the ability to adapt to changing conditions and maximize it's productivity with the resources that it has.

There are several weeks left in the growing season and the soybean plant still has time to take advantage of more favorable growing conditions to set and fill pods.

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As our local University of Missouri Agronomy Extension Specialist, Crook has been writing a column for the print edition Agriculture page for the past three years and we will now be sharing it on our web version. Crook has a bachelor and masters degree in agronomy from University of Missouri and received his doctorate in Agronomy from Kansas State University. He was in soybean variety development research for 22 years for various seed companies and has been Saline County's agronomy specialist for 10 years.
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