Understanding Growth Regulator Herbicide Injury

The plant growth regulator herbicides (PGR) are classified as Group 4 herbicides with sites of action (SOA) of 4 (synthetic auxin) and 19 (inhibitor of indoleacetic acid transport).^1,2 The chemistries are important for broadleaf weed control with uptake usually occurring through the foliage, though root uptake can occur. Upon uptake, the herbicides can move upward and downward through the plant’s xylem and phloem transportation channels to areas of new plant growth. Several herbicide families and active ingredients can be found in PGR herbicides, such as:

• Phenoxy carboxylic acids (2,4-D; 2,4-DB; MCPA; MCPB; MCPP)
• Benzoic acids (dicamba)
• Pyridine carboxylic acids (clopyralid, fluoxypyr, aminopyralid, picloram, triclopyr, aminocyclopyrachlor)
• Quinoline carboxylic acids (quinclorac)
• Semi-carbazones (diflufenzopyr)

The herbicides within these families control weeds by disrupting several plant growth processes including:

• Protein synthesis
• Cell division
• Cell enlargement
• Respiration

Regardless of herbicide group and family, the label for a respective herbicide MUST BE READ AND FOLLOWED. The label is the law. Plant growth regulator herbicides have the potential to injure crops by misapplication, drift, spray tank contamination, application during certain environmental conditions, and application when a crop is in a rapid growth phase. Other herbicides, viruses, insects, and environmental conditions can mimic PGR injury symptoms, so it is important to consider many potential causes for any observed symptoms.

Most PGR herbicides cause similar injury symptoms on broadleaf plants.³ The intensity of symptoms and potential for yield loss can depend on the herbicide, level of exposure, crop, crop growth stage, and certain environmental conditions such as drought or temperature inversions. Injury may be slight at low exposure to death at high levels of exposure. The potential for yield loss increases when susceptible plants are exposed to a PGR herbicide close to the plant’s reproductive growth stage (flowering or later).⁴

Because PGR herbicides act on meristematic tissue, injury symptoms generally appear on the youngest leaves, which can be cupped, crinkled, puckered, strap-shaped, stunted, and/or malformed. Injured leaf veins can appear parallel rather than netted.⁴ Additionally, plant stems can become bent, twisted, brittle, and may have shortened internodes.⁴ Stem twisting and leaf curling can occur within a few hours of exposure. After low level exposure, plant growth usually resumes even though the visual symptoms remain. With high levels of exposure, chlorosis can develop within a few days and result in leaf droppage, shoot tip death, and stem dieback.

Environmental conditions during and following a PGR application can increase the potential for PGR injury to susceptible crops and other vegetation. Wind speed and direction must be determined prior to spraying a PGR, as fine spray droplets can drift and be moved with the wind to non-targeted adjacent fields and surroundings. Temperature inversions occur after the sun sets and is no longer warming the earth’s surface. Instead of cooler air rising, as it does during the day, it settles closer to the surface and ceases the daily cycle of upward movement. Spray particles can then become suspended in the air and be pushed by a horizonal wind to non-target areas.⁵

Very small amounts of PGR herbicides remaining in tanks, lines, and nozzles can cause PGR injury to susceptible plants directly or by drift. Sprayers—as well as equipment used in transport, handling, and mixing—must be thoroughly and properly cleaned prior to switching to non-PGR herbicides.

Grass crops that are generally tolerant to PGR herbicides can display symptoms of a PGR application, particularly if the application occurs when the plants are growing rapidly. Since PGR herbicides are synthetic auxins, they promote rapid plant growth similar to naturally occurring auxins within the plant. Injury symptoms of PGR herbicides may be seen as fused brace roots in corn and sorghum (Figures 1 and 2), bent stalks (Figure 3), rolled leaves, and greensnap (Figure 4). Injury symptoms have a higher potential for occurrence if the herbicides are applied when the plants enter a rapid growth phase which is typically around V5 growth stage. The rapid growth can cause an increase in stalk brittleness and—along with other agronomic factors that favor fast growth such as adequate moisture, warm temperatures, and good fertility—increase the potential for greensnap to occur, especially during high wind events. A few corn products may be genetically more prone to greensnap and should be used with caution if PGR herbicides are in the production plan. Some PGR herbicides may contain safeners to help reduce the potential for crop injury.

In general, dicamba causes soybean leaves to turn up and become cupped (Figure 5) and 2,4-D causes soybean leaves to become long and narrow (strapping, Figure 6). Soybean plants are more sensitive to dicamba (Figure 5) than 2,4-D and cotton is more sensitive to 2,4-D (Figure 7). Sugarbeet plants are quite sensitive to PGR herbicides; however, clopyralid is labeled for use on sugarbeet but can cause injury at high rates under warm and moist environmental conditions.

Agronomic Incidents that Resemble PGR Injury

Certain environmental conditions can cause plants to have symptoms that mimic PGR injury. As an example, rapidly growing corn plants (around V5) subjected to cold nighttime temperatures followed by rapid warming can develop PGR exposure-like symptoms with thick leather-like leaves that can roll, twist, and bend over (Figure 8). When the leaves unfurl, they may have a crinkled appearance and be yellow because of the lack of photosynthesis (Figure 9).

Meristematic growth inhibitor herbicides can cause cupped or crinkled leaves and a drawstring appearance (leaf tip is pulls inward, Figure 10).

Protoporphyrinogen oxidase (PPO) inhibitor herbicides can cause existing leaves to expand, crinkle, and have a burnt appearance soon after foliar application (Figure 11).⁶ New leaves appear normal.

Bean pod mottle virus and soybean mosaic virus can cause leaves to develop crinkles and bumps (Figures 12 and 13). Leaf veins don’t grow together as they do with PGR injury. Broadleaf weeds are not susceptible to the diseases and therefore lack the symptoms.

Feeding by aphids can cause leaf cupping. Look on leaf undersides for aphids (Figure 14).

Severe spider mite feeding can result in cupped and deformed leaves (Figure 15).

Sources

¹Senseman, S. 2017. Herbicide site of action (SOA) classification list. Weed Science of America. https://wssa.net/wp-content/uploads/WSSA-Herbicide-SOA-20170403.pdf

²Manuchehri, M. 2017. Herbicide how-to: understanding herbicide mode of action. Oklahoma State University Extension. PSS-2778. https://extension.okstate.edu/fact-sheets/herbicide-how-to-understanding-herbicide-mode-of-action.html

³Al-Khatib, K. Herbicide symptoms. UCIPM. University of California Division of Agriculture and Natural Resources. http://herbicidesymptoms.ipm.ucanr.edu/

⁴Plant clinic fact sheet: Plant growth regulator herbicide injury. University of Illinois Extension Plant Clinic. https://extension.illinois.edu/sites/default/files/factsheet_on_pgr_herbicide_injury.pdf

⁵Ward, M. 2017. 8 things about temperature inversion you should know now. MissouriRuralist, FarmProgress. https://www.farmprogress.com/marketing/8-things-about-temperature-inversion-you-should-know-now.

⁶McGrath, C. 2017. Part 2 of diagnosing dicamba injury. FarmProgress. https://www.farmprogress.com/soybean/part-2-of-diagnosing-dicamba-injury

Web sources verified 2/13/25.

1119_139308