Thistle plants growing in a pot in the greenhouse that are twisted and curledCrinkled and deformed bean leaf in the foreground of a garden photoWeed with large leaves growing in bare soil; some of the leaves are curling at the edgesclose up photo of pine needles that are bent and curledPhoto of a row of zucchini plant growing in a crop field; the plants in the foreground are lighter green and showing symptoms of curling leavesPhoto showing a grapevine with large green leaves on the bottom and smaller distorted leaves on topClose up photo of wheat heads that are deformed

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Group Number

4

Where Used

Agricultural and horticultural settings, mainly to selectively control broadleaf weeds.

Common Active Ingredients

2,4-D, aminopyralid, clopyralid, dicamba, fluroxypyr, and picloram.

Effect on Plant

Herbicides in this mode of action mimic auxin, a plant hormone that regulates many aspects of growth. Synthetic auxin herbicides bind to hormone receptors in plant cells and cause a chain of events within the plant that lead to rapid and uncontrolled growth. Synthetic auxin herbicides are systemic, meaning they
move both from roots to shoots and shoots to roots.

Injury Symptoms

Typical symptoms include various growth abnormalities. Twisting, cupping, and curling of leaves, stems, and twigs on broadleaf plants is common (Figures 11–13). Conifers can also show symptoms of needle and shoot distortion (Figure 14). Chlorosis may occur with high exposure rates (Figure 15). Leaves may show symptoms of clear or translucent veins and parallel veins (Figure 16). Stems may crack or thicken. Leaves and shoots that emerge after exposure can show symptoms of stunting and distortion. Though these herbicides are usually selective for broadleaf plants, monocots like wheat can show symptoms of epinasty and distortion in certain situations (Figure 17).

 
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