The growth regulators include phenoxy acetic acids, benzoic acids, and the pyridines. Growth regulator herbicides can act at multiple sites in a plant to disrupt hormone balance and protein synthesis and thereby cause a variety of plant growth abnormalities. Growth regulator herbicides control broadleaf weeds and can injure sugar-beet. Herbicides in this group can move in both the xylem and the phloem to areas of new plant growth. As a result, many herbicides in this group are effective on perennial and annual broadleaf weeds. Herbicide uptake is primarily through the foliage but root uptake may also occur
1. Phenoxy Acetic Acids
a. Use: 2,4-D for small grains, corn, grass pastures, and non--cropland MCPA for small grains and grass establishment. 2,4-DB for alfalfa and soybean
b. Injury Symptoms: All phenoxy acetic acids produce identical symptoms in sugarbeet. Sugarbeet leaves will be flat on the ground within a few hours after exposure and leaves may remain more prostrate than normal for the rest of the growing season if injury is severe (Photo 1). Leaf petioles exhibit twisting, also called epinasty (Photo 2). Sugarbeet exposed to phenoxy acetic acids in the cotyledon to early four-leaf stage may develop fused petioles and a symptom called ``celery stalking" or "trumpeting" (Photo 3). Phenoxy acetic acid herbicides on larger sugarbeet will not produce "celery stalking." New leaf growth generally will resume at about six to 10 days after exposure to phenoxy acetic acids. New leaves often will be malformed with crinkled leaf margins, parallel veins, or leaf strapping. Rapid initiation of new leaf growth suggests less sugarbeet injury and less sugarbeet yield loss than a delayed initiation of new leaf growth. Sugarbeet injured with phenoxy acetic acids should be processed immediately after harvest because injured sugarbeet roots do not store as well as non-injured roots.
c. Site of Action: Specific site(s) unknown, believed to have multiple sites of action.
2). Sugarbeet exposed to phenoxy acetic acids in the cotyledon to early four-leaf stage
may develop fused petioles and a symptom called ``celery stalking" or
"trumpeting" (Photo 3). Phenoxy acetic acid herbicides on
larger sugarbeet will not produce "celery stalking." New leaf growth generally
will resume at about six to 10 days after exposure to phenoxy acetic acids. New leaves
often will be malformed with crinkled leaf margins, parallel veins, or leaf strapping.
Rapid initiation of new leaf growth suggests less sugarbeet injury and less sugarbeet
yield loss than a delayed initiation of new leaf growth. Sugarbeet injured with phenoxy
acetic acids should be
processed immediately after harvest because injured sugarbeet roots do not store as well
as non-injured roots.
c. Site of Action: Specific site(s) unknown, believed to have multiple sites of action.
2. Benzoic Acids
a. Use: Dicamba (Banvel) for corn, wheat, oats, sorghum, pastures, and noncropland.
b. Injury Symptoms: Banvel causes sugarbeet injury that is vis-ually indistinguishable from phenoxy acetic acid injury (Photo 4). Banvel may have sufficient residual in the soil to reduce emergence and to injure emerging sugarbeet. Emerging seedlings may be twisted, and development of the first true leaves may be inhibited (Photo 5). Banvel residual in soil can cause the "trumpeting" symptom (Photo 6) which also can be caused by early postemergence exposure to phenoxy acetic acid herbicides.
c. Site of Action: Specific site(s) unknown, believed to have multiple sites of action.
3. Pyridines
a. Use: Clopyralid (Stinger) for small grains, sugarbeet, corn and grass pastures. Picloram (Tordon) for non-cropland, small grains, and grass pastures. Triclopyr for non-cropland and grass pasture.
b. Injury Symptoms: Pyridine injury symptoms are very similar to the phenoxy acetic acids or Banvel. Stinger is registered on sugarbeet but can injured sugarbeet at high rates with a warm, moist environment that favors phytotoxicity. The leaves lay flat and the petioles exhibit epinasty from Stinger just as from phenoxy acetic acids or Banvel (Photo 7). Also, leaves may become more strap-shaped than normal. However, Stinger injury often causes leaves to roll upward from the edges (Photo 8). Leaf rolling is caused to a greater extent by clopyralid than by phenoxy acetic acids or Banvel.
c. Site of Action: Specific site(s) unknown, believed to have multiple sites of action.