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Practical Weed Science for the Field Scout: Corn and Soybean

Growth regulators

Injury from dicamba can cause stems to form callus tissue. Snap bean shown.
 

Early applications of dicamba + atrazine with a sulfonyl-urea herbicide and methylated seed oil resulted in this severe case of onion-leafing on corn.
 

Leaf cupping caused by dicamba drift or dicamba-contaminated spray tank. Sometimes the mottled appearance can be confused with certain viruses of soybean.
 

Stalk twisting and leaning after dicambe.

Green snap caused by high winds shortly after dicamba applications.
 

Dicamba and dicamba-containing products can also cause brace root fusion in corn with late postemergence applications.
 

Postemergence applications of dicamba can sometimes cause corn leaf rollling and mimic drought stress.
 

"Draw-stringed" leaves can also occur when proper rotational intervals are not followed with dicamba or clopyralid (Stinger, Hornet).
 

Benzoic acids and pyridines chemical group

Herbicides

• Dicamba (Banvel, Clarity, Distinct, Status, in Marksman) on corn, sorghum, wheat, pastures and turf
• Clopyralid (Stinger, in Hornet) on corn, pastures, and turf
• Triclopyr (Garlon/Rely) on grass forages/rangeland, and rice
• Fluroxypyr (Starane) on wheat
• Aminopyralid (Milestone) on pastures

Mode of action and characteristics of activity

• Hormone type
• Primarily annual and perennial broadleaf weed control, translocates upward in xylem and to growing points and roots in phloem, mobile in soil, some soil persistence
• Dicamba has vapor drift potential

General symptoms

• In broadleaf plants, necrosis (browning) of terminal meristematic areas, stem twisting and epinasty, early petioles turned down, and leaves cupped upward
• Broadleaf plants may exhibit more cupping than strapping (veins in leaf becoming parallel) of leaf tissue
• Corn may be onion-leafed (leaves do not unroll) and have brace root damage as well as missing kernels on cob

Injury from 2,4-D on soybeans appears as strapped leaves, irregular bending in the stems, and formation of callus tissue on the stem.
 

Injury from Harness + 2,4-D tankmixes on spike-stage corn.
 

Soybean injury can also occur when proper intervals are not followed after burndown with 2,4-D.
 

2,4-D injury on corn appears as brittle stems, rolled up leaves (onion-leafing) and fused brace roots.
 

Phenoxy acetic acid chemical group

Herbicides

• 2,4-DB (Butyrac — soybean, alfalfa)
• 2,4-D (corn, grain sorghum, rice, small grains, grass forages)

Mode of action and characteristics of activity

• Hormone type
• Annual and some perennial broadleaf weed control
• Translocates upward in xylem and to growing points and roots in phloem
• Short soil persistence
• Some vapor drift potential

General symptoms

• Abnormal growth responses — In broadleaf plants, youngest leaves are strapped, cupped upward and twisted
• May be brittle, branched, or callus tissue may develop on stems
• Corn plants exhibit onion-leafing, fused brace roots, stalk bending, brittleness, and missing kernels on cob

IPM1007, revised November 2009

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