Diagnosing Herbicide Drift Injury on Corn By Kevin Bradley

Photos by Kevin Bradley Figure 1

I've received several calls recently regarding corn that has been injured as a result of drift from a herbicide application made on a nearby field. Due to the widespread use and adoption of Roundup Ready soybean, most of these cases involve glyphosate injury to non-Roundup Ready corn hybrids. However, there are a variety of other herbicides that can cause significant injury on corn if drift occurs. The following article provides some tips for diagnosing injury to corn as a result of drift from other non-labeled herbicides.

Overall, corn that has been injured as a result of glyphosate drift or tank contamination will usually have a stunted appearance with chlorotic areas on the newest emerging leaves. Perhaps the most characteristic symptom of low levels of glyphosate drift or tank contamination is distinctive chlorotic "bands" on the newest corn leaves emerging from the whorl (Figure 1). This is a result of glyphosate coming into contact with these leaves while they are rolled up within the corn whorl. Once these leaves emerge from the whorl, these bands can usually be observed in a distinctive pattern around the plant. These bands also reveal where and when the glyphosate contacted the plant.

Figure 2

Depending on the severity of the glyphosate dose received, a variety of injury symptoms may be observed. For example, in some cases the whorl and newest leaves may be bleached to the point of being white (Figure 2). Additionally, glyphosate may cause some "purpling" of the corn leaf midribs under certain conditions, but does not typically cause entire leaves or leaf margins to turn purple as is the case with phosphorous deficiency. In cases where glyphosate tank contamination has occurred, injury symptoms may be Diagnosing Herbicide Drift Injury on Corn especially variable as several herbicides (including small amounts of glyphosate) and modes of action have been applied at the same time. The degree and significance of the corn injury may be determined by noting the amount of chlorosis in the newest leaves emerging from the corn whorl.

Although it is much less common, corn may also be injured as a result of drift or tank-mix contamination of glufosinate, which is sold as Liberty or Ignite. On non-Liberty Link corn varieties, glufosinate injury will appear within a few days after application. Very small, water-soaked lesions may be observed within just one day after application. Affected plants will turn yellow and then necrotic depending on the severity of the drift or tank-mix contamination. Corn leaves often have bleached white striped areas that may sometimes be confused with frost damage (Figure 3).

Figure 3

Figure 4

Corn may also show signs of injury as a result of drift from paraquat, which is sold under the trade name of Gramoxone Max, Cylcone Max, and Boa. This typically occurs on emerged corn in areas where a burndown application of paraquat has been applied to a nearby field that will be no-till planted to corn or soybean. Initial symptoms of paraquat injury can be observed within a few hours after application especially during hot sunny days. Paraquat destroys cell membranes so within a few hours after contact with paraquat, corn leaves will have indiscriminate, grayish water-soaked lesions that are a result of the cellular constituents leaking out of the plant cell membranes (Figure 4). Within a few days after application, these lesions will progress and appear as bleached white or necrotic spots wherever the herbicide came into contact with the corn foliage (Figure 5). The severity of the injury will be dependant on the degree of paraquat drift that has occurred, but this "speckling" is usually minor and yield reductions will only occur under severe cases.

Figure 5

Another class of herbicides that may cause injury to corn as a result of drift from a nearby soybean field is the graminicides, or grass herbicides. These include the herbicides fluazifop-P (Fusilade), quizalofop-P (Assure II), fenoxaprop (in Fusion), clethodim (Select), and sethoxydim (Poast Plus). These products have 'regained' some of their usefulness in recent years due to the emerging problem of controlling volunteer Roundup Ready corn in Roundup Ready soybeans. All of these products have a similar mode-of-action and will cause essentially identical symptoms on affected corn plants. Corn that has been injured as a result of drift from one of these herbicides will have newer leaves that become distinctly reddish in appearance (Figure 6). Another key symptom is that new leaves in the whorl will separate easily from the remainder of the plant and will become necrotic and rotten at the point of the leaf attachment (Figure 7).

Figure 6

Figure 7

For more information and pictures pertaining to the diagnosis of corn injury symptoms as a result of drift, tank contamination, or misapplication of herbicides, see the MU Herbicide Symptomology website at http://www.psu.missouri.edu/agronx/weeds/Web%20Resources/herbinjsymptoms/muhrbinj.html.

Kevin Bradley
573-882-4039