Herbicide and Nonherbicide Injury Symptoms on Spring Wheat and Barley

Amino Acid Synthesis Inhibitors
The amino acid synthesis inhibitors include the sulfonylureas, imidazolinones, and amino acid derivatives herbicide families. Amino acid synthesis inhibitors act on a specific enzyme to prevent the production of specific amino acids, the key building blocks for normal plant growth and development (Figure 1). Sulfonylurea and imidazolinone herbicides prevent the production of three essential branch-chain amino acids by inhibiting one key plant enzyme. This key plant enzyme is called acetolactate synthase (ALS) or acetohydroxy acid synthase (AHAS). The amino acid derivative herbicides inhibit the production of three essential aromatic amino acids by inhibiting another key plant enzyme, called SEPSP synthase. In general, injury symptoms are slow to develop (1 to 2 weeks) and include stunting or delayed plant growth that leads to eventual death of the plant. Herbicides in the sulfonylurea and the imidazolinone families can move in both the xylem and phloem to areas of new growth and can be taken up through plant foliage and roots. The amino acid derivative herbicides are nonselective and the site of uptake is the plant foliage. Herbicides in this family move via the phloem to all parts of the plant; these are excellent annual weed control herbicides and are active on perennial weeds as well.

Figure 1. Enzymes function as steps in biological processes. Enzymes are also extremely specialized in their function. As a result, different enzymes are involved with the many different biological processes that occur within a plant. Some herbicides can stop specific enzymes from functioning, resulting in a disruption of specific plant processes; this often leads to the death of the plant. This herbicide-enzyme relationship is very specific and any chemical modification of the herbicide or enzyme can eliminate herbicidal activity. Reprinted with permission from Herbicide Mode of Action, J. Gunsolus and W. Curran, NCR Extension Publication 377, University of Minnesota Extension Service, St. Paul, 1996. PLANTS ARE HEALTHY PLANTS ARE KILLED

Imazamethabenz (Assert)
Application Timing (wheat and barley): Postemergence after 2-leaf stage but before the second node is visible. Wheat and barley have good tolerance to Assert, and injury is uncommon. Injury may occur under adverse weather conditions and high application rates.
Injury Symptoms: Slight discoloration (yellowing) of youngest leaves and delayed growth.
Site of Action: ALS inhibitor.

Chlorsulfuron (Glean, Finesse)
Application Timing (wheat and barley): Postemergence from the 2-leaf up to the preboot stage. Spring wheat and barley have good tolerance to Glean, and crop injury is uncommon.
Injury Symptoms: Injury symptoms that could occur are: a reduction in tiller number, spike number, plant stand, and kernel weight; leaf chlorosis with yellow striations occurring on youngest leaves; stunted plants and distorted growth. Do not tank mix with malathion insecticide as crop injury may result.
See photos 1 and 2.
Site of Action: ALS inhibitor.

Photo 1	Photo 2
Imidazolinones or sulfonylureas can cause yellowing or stunting of small grain. Injury is generally temporary.

Metsulfuron (Ally, Finesse)
Application Timing (wheat and barley): Postemergence from the 2-leaf to the preboot stage. Spring wheat and barley have good tolerance to Ally and crop injury is uncommon.
Injury Symptoms: See injury symptoms under chlorsulfuron.
Site of Action: ALS inhibitor.

Triasulfuron (Amber)
Application Timing (wheat and barley): Preplant incorporated or preemergence; postemergence from the 2-leaf to the preboot stage. Spring wheat and barley have good tolerance to Amber, and crop injury is rare.
Injury Symptoms: See injury symptoms under chlorsulfuron.
Site of Action: ALS inhibitor.

Thifensulfuron + Tribenuron (Harmony Extra)
Application Timing (wheat and barley): Postemergence from 2-leaf stage but before the flag leaf is visible. Spring wheat and barley have good tolerance to Harmony Extra.
Injury Symptoms: See injury symptoms under chlorsulfuron.
Site of Action: ALS inhibitor.

Tribenuron (Express)
Application Timing (wheat and barley): Postemergence from 2-leaf stage, but before the flag leaf is visible.
Injury Symptoms: See injury symptoms under chlorsulfuron.
Site of Action: ALS inhibitor.

Prosulfuron (Peak)
Application Timing (wheat and barley): Postemergence from the 3-leaf stage, but before the second node is detectable in stem elongation.
Injury Symptoms: See injury symptoms under chlorsulfuron.
Site of Action: ALS inhibitor.

Glyphosate (Roundup Ultra)
Labeled Application Timing (wheat and barley): Preplant and preemergence application in reduced and no-tillage fields. Labeled as preharvest application for perennial weed control. Misapplication, off-target drift, or tank residual contamination may occur.
Injury Symptoms: Youngest leaves near the growing point slowly become chlorotic and later die. Chlorotic segments may appear in heads after exposure to very low rates.
See photos 3 and 4.
Site of Action: EPSP synthase enzyme.

Photo 3 Roundup injury on spring wheat. Youngest leaves near growing point slowly become chlorotic and die

Photo 4 Roundup drift injury on spring wheat.

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