Synthetic Auxins
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).