Pea plants growing in a crop field with yellow leaves and stems at the top of the plantPhoto showing the top of a sunflower plant with small yellow leaves.Small, stunted pea plants growing in a crop field with a camera lens cover in the foregroundSmall, stunted lentil plants growing in a crop field with a camera lens cover next to themThe top of a raspberry plant with small narrow leavesTree branch with leaves that are smaller and narrower than usual.

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WSSA Group Numbers

2 and 9

Where Used

Many settings including cropland, rangeland, residential areas, and aquatic systems.

Common Active Ingredients

Group 2: imazapyr, metsulfuron, propoxycarbazone; Group 9: glyphosate

Effect on Plant

Amino acid synthesis inhibitor herbicides prevent specific enzymes from producing certain amino acids in younger, actively growing plant tissue. Amino acids are needed to make proteins and are crucial to growth and development of plants. There are two distinct groups, meaning there are different amino acid synthesis enzymes that are affected, but they have similar injury symptoms. Group 2 herbicides affect the acetolactate synthase enzyme and are commonly called ALS inhibitors. The group 9 herbicide is glyphosate which affects a different enzyme called EPSP synthase (5-enolpyruvylshikimate-3-phosphate). Overall, herbicides in this mode of action are systemic and are translocated within the plant to growing points. They can be either nonselective (affecting grasses and broadleaves) or have selective activity (affecting only certain plants).

Injury Symptoms

Grasses and forbs: Growth usually stops after application and plants appear stunted compared to non-symptomatic plants. Symptoms include chlorosis and eventually necrosis that starts in the new growth (Figures 3–4). Shoot growth may stop or appear stunted and distorted (Figures 5–6). Because amino acids are needed in new growth and these herbicides stop that synthesis, injury appears in new growth first. Plants usually do not show symptoms for several days after exposure, and they may take weeks to die from an application.


Trees and shrubs: Leaves may be narrow and chlorotic (Figure 7). If exposure is severe enough, branches may dieback starting from the tip of the branch. Growth from buds below the dieback portion of the branch may be chlorotic and distorted with excessive sprouting called “witch’s broom” (Figure 8). There is also evidence that glyphosate injury can cause increased risk of winter injury in the form of trunk splitting or cracking. Woody plants may show symptoms for a year or more after exposure.

 
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