How to Diagnose Herbicide Injury

Determining if non-target herbicide injury is the cause of a plant health issue involves assessing multiple types of evidence. The investigator sees the end result of an issue and works backward to try to determine the cause. Diagnosing herbicide injury requires accurately describing symptoms and finding as much information about site history as possible. In some cases, the cause of the problem may be very straightforward, and in other cases more  investigation, research, and critical thinking is needed. There are four important questions to ask when diagnosing herbicide injury.

What are the Plant's Symptoms?

First, determine what the symptoms of concern are. Describe symptoms (i.e. epinasty, stunting, chlorosis, necrosis, etc.) and where they appear on the plant (i.e. old growth or new growth, leaves, stems, roots, etc.). See the glossary of terms at the end of this guide for descriptions. Compare the symptomatic plant to “healthy” plants from the same area. Also look at nearby fields or home landscapes. If similar symptoms are occurring in areas managed by different people, the issue could be environmental or biotic rather than herbicide related. Also check if multiple plant species have the symptoms. If a single species is symptomatic among multiple species, then symptoms may not be herbicide related.

What is the Spatial Pattern of Symptoms in the Field or Landscape?

Often there are distinct spatial patterns to herbicide injury, and it is important to understand how injured plants are distributed. Examine whether symptomatic plants display a pattern, for example they may be along the edge of a field or randomly scattered throughout the field. In this example, symptoms occurring at the edge of a field may indicate herbicide drift, while symptoms scattered throughout a field may suggest a soil-applied herbicide issue. Another example is sprayer overlap injury which can look like parallel streaks of symptomatic plants running through a field.

Can Symptoms be Explained by Sometheing Other Than Herbicide?

Rule out insect, disease, environmental, or other plant health problems as the cause of symptoms. For example, distorted leaves on newer plant growth can be caused by weather events such as frost or hail, or insects like thrips, aphids, or leaf-roll caterpillars. Chlorosis and necrosis caused by droughtstress or nutrient deficiency can also resemble herbicide injury. Keep in mind that some plants naturally have characteristics that may look like herbicide symptoms. For example, variegated cultivars of certain plants or those that have red foliage can mimic certain leaf symptoms, while weeping cultivars of certain plants can appear to have epinasty symptoms. This step may require consultation with a local Extension office.

What is the Mode of Action of the Herbicide Suspected to be Causing Symptoms, and Do Symptoms Match the Suspected Mode of Action?

Information needed if the suspected cause of injury is a specific herbicide application:

If available, background information about the herbicide application is very important. Obtain a detailed description of the treatment from the applicator if possible.

• Determine which herbicide products were used and look at the active ingredient(s).
• Find the mode of action of the active ingredient that was applied and determine if plant symptoms are consistent with that mode of action. Herbicides kill, injure or prevent plants from growing by interfering with certain plant functions. The mode of action is defined as how an herbicide injures the plant or disrupts plant growth. Examples include disrupting photosynthesis or blocking the production of certain amino acids.
• This guide uses the Weed Science Society of America (WSSA) site of action group number and the general mode of action to group herbicide injury. The WSSA group number is often listed on the herbicide label, and can also be found at https://www.greenbook.net/
• Determine date of application and weather at time of application. Abnormally hot, cold, wet, or dry weather around the time of application can affect herbicide performance and plant tolerance to herbicides. Weather conditions can also influence the risk of herbicide drift. Product labels often provide information on how weather conditions prior to and following an application can influence herbicide performance.
• If applicable, find out what other products were in the tank mix including insecticides, fungicides, fertilizers, and adjuvants. In some cases, herbicides can interact with other pesticides or adjuvants to increase the risk of non-target injury. Again, the label often describes how a product’s performance can be affected when mixed with other products.

Information needed if the suspected cause of injury is carryover, or residual, from a previous herbicide application:

In the case of herbicide carryover, injury may be observed in plants from a past application. In some cases, herbicides applied several years prior to the observed injury may be the cause.

• Ask the producer or land manager for herbicide records from previous seasons. Depending on the case, up to three or four years of records may be needed. Review product labels to determine plant-back intervals for herbicides previously applied and symptomatic plants.
• Ask if any soil amendments like compost or manure were applied, or if topsoil was recently brought to the site. Carryover can happen when contaminated materials are moved from one area to another where they damage non-target plants. For example, manure contaminated with persistent synthetic auxin herbicides can cause herbicide injury symptoms in garden plants.

If no known herbicide application occurred, determine if drift from another property is possible.

Additional Resources for Diagnosing Herbicide Injury

If herbicides are suspected to be the cause of the issue from a visual analysis and review of site history, soil and/or plant tissues can be analyzed for herbicide residues for a fee at a laboratory. Such an analysis may be necessary to confirm a diagnosis. If a person suffers loss or damage of property from non-target herbicide injury, the Montana Department of Agriculture (MDA) is available to help investigate. To file a report of loss, contact your local MDA field office.