Corn: Herbicide Injury

Excerpt from Agronomy Guide for Field Crops (Chapter 3)

Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

1. Introduction
2. Specific Types of Herbicide Injury
   • Dinitroaniline
   • Chloroacetamide
   • Bleaching Herbicides
   • Auxinic Herbicides and Other Products Containing These Herbicides
   • Bromoxynil
   • Acetolactase Synthase (ALS) and Other Products Containing These or Similar Herbicides
   • ACCase Inhibitors
   • Glufosinate Ammonium
   • Glyphosate Injury
3. Updates on Corn: Herbicide Injury
4. Related links...

Introduction

All herbicides have the potential to cause crop injury. Some soil applied (preplant or preemergent) herbicides can be splashed up on the leaves to cause injury, or the roots can take up the herbicide. With post-emergent herbicide products, most injury results at or shortly after application. Spray from the target crop onto a neighbouring non-target crop can also cause injury when the non-target crop is susceptible to the herbicide being applied. Spray overlaps are also common places where crop injury from herbicides can occur.

Stage of growth, crop stress, weather conditions, variety, tank mix partners and adjuvants can all affect the amount and severity of crop injury. When the target crop is under stress, it is sometimes not able to metabolize the herbicide fast enough to avoid injury. The type of herbicide (mode of action) is a major factor in how herbicides affect crops. In general, while contact herbicide injury may look worse, systemic herbicides will have longer-lasting and more detrimental injury, which may be more severe to the crop. Always read the label for information on how to reduce the risk for herbicide injury.

For more information, see the OMAFRA Factsheet, Herbicide Mode of Action Categories, or to order Order No. 00-061, or visit the OMAFRA Web site at www.omafra.gov.on.ca/english/crops/.

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Specific Types of Herbicide Injury

| Dinitroaniline | Chloroacetamide | Bleaching Herbicides | Auxinic Herbicides | Bromoxynil |
| Acetolactase Synthase (ALS) | ACCase Inhibitors | Glufosinate Ammonium | Glyphosate |

Refer to the OMAFRA Factsheet, Herbicide Mode of Action Categories, or to order the factsheet Order No. 00-061, or Publication 75, Guide to Weed Control, for herbicide trade names.

Dinitroaniline (e.g., trifluralin, pendimethalin)

Dinitroaniline injury on corn caused by high rates or slow breakdown is shown . Short roots with thickened root tips or stunted top growth with purplish leaves are common symptoms.

Plate 40. Dinitroaniline injury on corn.

Chloroacetamide (e.g., dimethenamid, flufenacet, s-metolachlor)

Plate 41 displays chloroacetamide injury on corn. Leaves are shortened, puckered and the coleoptile is twisted, with the leaves unable to unfurl properly. Plant stress can reduce the plant metabolism and ability to breakdown herbicide and causes more injury.

Plate 41. Chloroacetamide injury on corn.

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Bleaching Herbicides (e.g., isoxaflutole, clomazone)

Plate 42 shows bleached leaves on corn caused by isoxaflutole at high rates. Plants may emerge normally and then show symptoms at the 2-4-leaf stage. Symptoms may also occur after a rain. Leaves will appear yellow to white and are usually more pronounced near the midrib. Purpling may appear at the leaf margin or at the base of the whorl.

Figure 42. Isoxaflutole injury. Bleached leaves on corn caused by isoxaflutole at 4X rate, normal field rate and check (no herbicide).

Auxinic Herbicides and Other Products Containing These Herbicides (e.g., 2,4-D, MCPA, dicamba, clopyralid)

Plate 43 shows the "onion leaf" symptom on corn caused by high rates of dicamba (this can also occur on sensitive hybrids or when applied too late).

Plate 43. "Onion leaf" symptom on corn caused by auxinic herbicides.

Dicamba injury causes twisted brace roots on corn. Brace roots are gnarled and tend to grow together and do not grow into the soil to support the plant, leading to potential lodging late in the season. High rates, sensitive hybrids or late applications can all cause these symptoms.

Plate 44. Dicamba injury causes twisted brace roots on corn.

Plate 45 shows the effect on pollination when sprayed too late.

Plate 45. Auxinic herbicide injury on ears of corn (example 2,4-D, dicamba).

Bromoxynil

Plate 46 shows bromoxynil injury on corn. Symptoms include a speckling of the leaf and browning on leaf tips and edges. Contact injury will show on leaves that are emerged at the time of treatment but not on new leaves emerging after treatment.

Plate 46. Bromoxynil injury on corn.

Acetolactase Synthase (ALS) and Other Products Containing These or Similar Herbicides (e.g., nicosulfuron, imazethapyr, chlorimuron-ethyl)

Sulfonylurea (ALS) injury on corn often appears as a "flash" on the leaf. Typically, corn outgrows this level of injury. Symptoms can include a yellow discolouration of the leaves, yellow flash on leaf (as shown), stunting of the plant, a delay in plant development and crinkled leaves or bleaching near the whorl. Onion-leafing or a purpling of stem or leaves may also occur. Some ALS herbicides may also reduce the root mass by shortening lateral roots. These herbicides should not be applied to corn that has been treated with an organophosphorus insecticide due to the increased risk of injury.

Plate 47. ALS injury on corn showing "flash" on leaf.

ACCase Inhibitors (e.g., sethoxdim, fluazifop-p-butyl, quizalofop-p-ethyl)

Group 1 graminicide (ACCase) injury on corn appears as intervenal chlorosis on newer leaves. Older leaves may show red or purple. The growing point turns brown and dies. The top of the plant can be easily pulled out of the whorl. New growth dies first, such that the plant appears to take considerable time to die after application.

Plate 48. Group 1 graminicide (ACCase inhibitors) injury on corn.

Glufosinate Ammonium

Plate 49 shows a susceptible corn plant injured by glufosinate ammonium herbicide adjacent to plants that are tolerant. Susceptible plants turn yellow and die. Injury occurs within the first few days after application.

Plate 49. Corn plants susceptible and tolerant to glufosinate ammonium.

Glyphosate Injury

Glyphosate drift on corn causes yellowing in whorl (some reddening on leaf edges) near the growing point. Leaves can become twisted, and plants will become stunted or die. Injury may take 1-2 weeks to develop and is not reversible.

Updates on Corn: Herbicide Injury

No updates available at this time.

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Related links...

• Weeds
• Corn
• Field Crop Protection Guide - Publication 812
• Guide to Weed Control - Publication 75

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