NC State Extension Publications

Problem

Herbicide injury caused by root-inhibiting herbicides such as dinitroanilines (including benefin, oryzalin, pendimethalin and trifluralin) and pyridine (such as dithiopyr).

Plant Entry

Root inhibitors are soil applied, preplant incorporated (ppi), at planting or post-transplant preemergence and assimilated by roots of germinating seeds (radical or seminal) and/or shoots prior to soil emergence. There is minimal to no translocation. Annual grasses are the primary target but some small-seeded broadleaf weeds are also controlled.

Crops injury may occur from soil residual carryover from previous crops, excessive doses from misapplication or inaccurate sprayer calibration or foliar exposure. Cool, wet conditions are conducive to injury to some crops by soil-applied dinotraniline herbicides.

Symptoms

Symptoms from root exposure:

  • Typically, sensitive plants either do not emerge or lodge shortly after emergence
  • Reduced root numbers
  • Swollen and stubby roots in grasses (mitotic inhibition)
  • May increase sensitivity to stress-related plant diseases

Symptoms on foliage and stems when contacted by spray applications:

  • Swollen and brittle stems in broadleaf plants (mitotic inhibition)
  • Foliar absorption may cause deformed and “puckered” new growth
  • When applied at or near flowering, the number of flowers can be reduced
Stem girdling

Stem girdling just at or below the soil line is often associated with stem swelling just above the soil line as seen in this photo of a pine seedling treated with oryzalin.

J. Neal

a lettuce seedling with deformed leaves

Leaf deformity on a lettuce seedling caused by pendimethalin.

R. Smith  CC BY-NC-ND - 4.0

a field planting of turnip plants with normal growth

Turnip crop with normal stand and plant growth.

K. Jennings  CC BY-NC-ND - 4.0

The turnip planting is thinned and plants are smaller

Turnip crop with stand thinning and stunted growth from Trifluralin at 3 pt/A 35DAT.

K. Jennings  CC BY-NC-ND - 4.0

turnips in harvest bins showing reduced yield from trifluralin

Turnip yield can be decreased by dinitroanaline herbicides such as trifluralin.

K. Jennings  CC BY-NC-ND - 4.0

Club-roots on a grass treated with prodiamine

Club-roots on a grass treated with prodiamine (a dinitroanaline herbicide). Swollen roots are still functional and with sufficient irrigation or rainfall such plants can survive and grow but lodge when roots cannot support the tops or die when soil dries.

J. Neal

Rudbeckia non-treated.

Rudbeckia non-treated.

J. Neal

Rudbeckia treated with prodiamine + isoxaben.

Rudbeckia treated with prodiamine + isoxaben.

J. Neal

Non-treated plant

Flowering of some bedding plants can be reduced by foliar application of root growth inhibitors. Non-treated plant.

J. Neal

Dithiopyr EW treated plants.

Flowering of some bedding plants can be reduced by foliar applications of root growth inhibitors. Dithiopyr EW treated plants.

J. Neal

Stand thinning in ornamental grass plant

Stand thinning in an ornamental grass plant bed resulting from an application of prodiamine.

Affected Plant Parts

Root inhibiting herbicide injury symptoms are often not observed as the roots of sensitive seedlings are quickly injured following emergence from the seed. Stems of sensitive plants that happen to emerged from the soil are weak and will soon lodge. Higher doses of dinitroaniline could present carryover for sensitive crops resulting in stand reduction and stunted growth. Some herbicides in these two families have up to 120 days half-life adding carryover concern. Stems of sensitive plants may be girdled at the soil line resulting in sudden wilting, stem breakage and death of treated plants. Foliar treatment can damage the youngest foliage present at treatment time, stunting bud development; older foliage and other buds are generally not affected.

Similar Problems

Root inhibitor injury may be confused with symptoms from:

  • Urea or high-salt fertilizers applied at planting can damage roots resulting in seedling death.
  • Stand reduction may resemble injury from parasitic nematodes or soil-borne diseases.
  • Similar stem girdling may be caused by cellulose synthesis inhibitor herbicides like indaziflam.
  • “Puckered” foliage may be similar to aphid damage, foliar damage from isoxaben, or early symptoms from synthetic auxin herbicides.

Herbicide Mode of Action Category

WSSA – 3
HRAC – K1

Useful Resources

North Carolina Agricultural Chemicals Manual

Southeastern US Vegetable Crop Handbook

Wolfpack Weeds

Weed Management in Nurseries, Landscapes & Christmas Trees Information Portal

Applied Weed Science: Including the Ecology and Management of Invasive Plants (3rd Edition), Merrill Ross & Carol Lembi, pages 168-169, 179-180, 279-283

Authors

Professor of Weed Science, Extension Specialist & Department Extension Leader
Horticultural Science
Professor Emeritus
Cobbleskill University
Assistant Professor
Horticultural Science

Publication date: May 10, 2016

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