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Are Growth Regulator Herbicides Causing Your Cupped Soybean Leaves?

Dean Riechers Dean Riechers
Assistant Professor
Department of Crop Sciences
(217) 333-4424
  Kevin Kelley Kevin Kelley
Graduate Research Assistant
Department of Crop Sciences
(217) 244-6882
Aaron Hager Aaron Hager
Extension Specialist
Department of Crop Sciences
(217) 333-4424

Several plant growth regulator (PGR) herbicides commonly used in corn for postemergence broadleaf weed control can injure soybeans at extremely low rates. These PGR herbicides include dicamba (found in Banvel, Clarity, Distinct, and premixes Northstar, Celebrity Plus, and Marksman), clopyralid (found in Stinger and premixes Accent Gold and Hornet), and 2,4-D.

There are several ways that a soybean crop can unintentionally come in contact with a PGR herbicide. Spray droplets can drift from a neighboring cornfield during application, although this source is usually easily traceable. PGR herbicides also can be directly applied to soybeans as residues remaining in a spray tank from previous applications to a cornfield. Spray additives and adjuvants may be involved in removing trace residues of PGR herbicides from the spray tank. In addition, a PGR herbicide can volatilize from where it was applied and move to a nearby soybean crop. Ester formulations of PGR herbicides, such as 2,4-D ester, are more volatile than salt (amine) formulations. Dicamba is also volatile, and soybeans appear to be especially sensitive to dicamba. As little as 1/10,000th of a field use application rate of dicamba can cause leaf cupping symptoms on newly emerged soybean leaves.

Many reported cases of soybean leaf cupping injury can be traced to one of these sources of exposure. Typical injury symptoms from these herbicides include leaf cupping, strapping, crinkling, parallel venation, and puckering. In addition to causing severe injury to newly emerging soybean leaves, PGR injury to soybeans also can lead to delayed maturity, decreased yield, and decreased germination of harvested soybean seed.

Leaf cupping injury from PGR herbicides
Figure 1. Leaf cupping injury
from PGR herbicides on newly
emerged soybean leaves.

In some cases, the injury observed in the field cannot be readily traced to a PGR herbicide and could have other causes. Some soybean viruses cause symptoms that can be mistaken for injury from PGR herbicides. Both Soybean mosaic virus and Bean pod mottle virus cause plants to be stunted and have crinkled or cupped leaves, similar to PGR herbicide-damaged soybeans. Also, it has been speculated that other stresses, such as postemergence herbicides normally used in soybeans or environmental stresses, may disrupt the natural hormonal balance in a soybean plant and induce growth regulator-like injury symptoms. However, no information in the literature directly supports these theories.

We are working on developing a laboratory assay to detect the presence of PGR herbicides in soybean leaves, based on their effects on gene expression levels and expression patterns. Using molecular techniques in the laboratory, we can fingerprint the expression of one or many genes in soybean leaves that have been treated with very low levels of the PGR herbicides. We hope to find a gene (or genes) that is expressed only in PGR-treated soybean leaves and will then use the expression of this gene as a molecular marker to diagnose the presence of the herbicide. We also can directly detect the presence of the two soybean viruses mentioned above using laboratory tests.

We have already found a gene that is expressed only in soybean leaves treated with each of the three PGR herbicides listed above, and we hope to find specific genes that can be used to distinguish among dicamba, clopyralid, and 2,4-D. This assay would allow for the diagnosis of the causal agent of soybean leaf cupping injury and could help determine if the cupped soybean leaves that are frequently observed in the field are the result of PGR herbicide exposure or one of the common soybean leaf viruses found in Illinois.

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