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Growth Regulator Herbicide Symptoms on Soybean (08/06/15)

Samples have come in showing classic growth regulator (GR) symptoms on soybean.

Growth Regulator Herbicide Symptoms on Soybean

Samples have come in showing classic growth regulator (GR) symptoms on soybean. In reviewing the field and chemical histories there was no indication of herbicide carryover, spray tank contamination, or droplet particle or vapor drift from the nearby area. This phenomenon has been seen certain years across the Midwest at this time of the year when soybean plants show accelerated growth. The leaf strapping and blistering from rapid soybean growth is classic symptoms that mimic growth regulator herbicides. Applied glyphosate which would normally not cause any expression of abnormal growth may also contribute to observed symptoms and may interact with environment and rapid plant growth to cause these GR type symptoms.

There are a few factors that can differentiate this from actually GR herbicide/plant interactions. Leaf strapping and blistering from rapid soybean growth would be observed mainly on the leaves. GR herbicides can also cause these symptoms but in addition may cause epinasty (bending and curving) of leaf petioles. Soybean varieties may also show varying levels of symptoms expression – one variety may show prominent symptoms while another may show no abnormal growth.

A “loose” rule of thumb for soybean symptoms from GR herbicides is:

2,4-D = leaf strapping, blistering and leaf petiole epinasty. Stem cracking and callus growth may form on stems.

Dicamba = leaf blistering and upward cupping.

Clopyralid = Fiddlenecked leaves and growing points and some leaf strapping.

This year many have seen growth reduction/stunting in addition to the growth regulator symptoms on soybean plants. This does not fit the normal expression of rapid growth symptoms. There may not be a good explanation for this excessive symptomology except some varieties seem more susceptible than others. I have observed the same stunting and rapid growth symptoms on soybeans in my research plots with no evidence of carryover or drift from a growth regulator herbicide.

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Rich Zollinger

Extension Weed Specialist

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