NC State Extension Publications

Problem

Herbicide injury caused from photosystem I (PS I) inhibitors including (bipyridilium herbicides) paraquat (Cyclone, Gramoxone) and diquat (Reglone, Reward). These are non-selective, contact-acting herbicides producing rapid necrosis of contacted plant tissues.

Symptoms

  • Non-selective necrosis of plant foliage.
  • Leaf water-soaking on all treated tissues within hours of treatment.
  • Necrosis of contacted foliage within 4 to 6 hours.
  • Spotting and necrotic lesions on fruit contacted by the spray droplets.
  • High doses applied to the stem of plants can cause stem cankers even in woody plants.
Paraquat applied POST to sweetpotato three days after treatment.

Paraquat applied POST to sweetpotato three days after treatment.

K. Jennings  CC BY-NC-SA - 4.0

Diquat spray drift on knockout rose foliage.

Diquat spray drift on knockout rose foliage, 1 week after treatment.

J. Neal  CC BY-NC-SA - 4.0

Diquat spray drift on knockout rose.

Diquat spray drift on knockout rose, 2 weeks after treatment. Note new growth emerging unharmed.

J. Neal  CC BY-NC-SA - 4.0

Diquat directed application to cryptomeria japonica.

Diquat directed application to cryptomeria japonica caused necrosis of contacted foliage but growth not contacted by the spray was unaffected.

J. Neal  CC BY-NC-SA - 4.0

Diquat spray drift on grape foliage.

Diquat spray drift on grape foliage.

B. Lassiter  CC BY-NC-SA - 4.0

Diquat spray drift on peach foliage.

Diquat spray drift on peach foliage, 2 weeks after treatment. These spots turned necrotic then "dropped out" leaving "shot holes."

J. Neal  CC BY-NC-SA - 4.0

Diquat spray drift on strawberry.

Diquat spray drift on strawberry, 2 weeks after treatment. Deep veins on the leaves acted as "troughs" where spray droplets collected resulting in what appears to be veinal necrosis.

J. Neal  CC BY-NC-SA - 4.0

Diquat spray drift on sweet potato foliage.

Diquat spray drift on sweet potato foliage, 2 weeks after treatment.

J. Neal  CC BY-NC-SA - 4.0

Diquat spray drift on sweet potato stem, 2 weeks after treatment

Diquat spray drift on sweet potato stem, 2 weeks after treatment.

J. Neal  CC BY-NC-SA - 4.0

stem canker on dogwood caused by paraquat

Excess doses of paraquat can cause bark damage to thin-barked trees such as this dogwood.

J. Neal  CC BY-NC-SA - 4.0

Plant Entry and Symptom Expression

Foliar applied, nontranslocated contact herbicides causing rapid destruction of cell membranes. PS-I inhibitor herbicides do not have soil residual activity. Photosystem I inhibitor injury symptoms are visible on all treated tissues within hours of application and plant death within one to two days. Increased speed of PS I activity in bright sunlight, high humidity and/or moist soil. Amount of tissue necrosis on treated plants depends on the amount of spray contact – leaf spot when small amounts of drift occur; necrosis of leaves and green stems with sufficient spray coverage.

Similar Problems

PS I inhibitor injury may be confused with symptoms from:

  • Bentazon and bromoxynil (PS II rapid action) cell membrane destruction.
  • PPO inhibitor herbicides (such as acifluorfen flumioxazin, and lactofen).
  • Glufosinate (glutamine synthetase).
  • High salt concentration resulting in desiccation.
  • Drought related leaf necrosis.
  • Excess concentration of crop oils, surfactants and other spray adjuvants.
  • Gasoline, diesel fuel and other petroleum products may also display contact herbicide symptomology.

Herbicide Mode of Action Category

WSSA – 22

HRAC – D

Authors:

Professor Emeritus
Cobleskill Univ.
Professor of Weed Science, Extension Specialist & Department Extension Leader
Horticultural Science
Assistant Professor
Horticultural Science
Extension Associate, Horticulture
Horticultural Science

Publication date: Sept. 10, 2015

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