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University of Illinois Extension

Photosynthesis (PSI and PSII) Inhibitor Herbicide Injury

Atrazine herbicide drift on pumpkin (cucurbit).

Atrazine herbicide drift on pumpkin (cucurbit).


Plants affected dependant upon herbicide used.

The list below of specific plants is NOT all inclusive. It reflects the authors' experience as well as University of Illinois herbicide injury research trial results.  


These herbicides all inhibit photosynthesis in susceptible plants.

Triazines:atrazine (numerous premixes), metribuzin (Sencor), and simazine (Princep).

Characteristics of activity:

  • Translocated in xylem.

Symptoms of injury:

  • Leaf margin burn and interveinal chlorosis.
  • Older leaves affected first

Benzothiadiazoles: bentazon (Basagran), Phenylpyridazines: pyridate (Tough), and Benzonitriles: bromoxynil (Buctril)

Characteristics of activity:

  • Limited translocation.

Symptoms of injury:

  • Necrotic spots, speckles chlorosis, or bronzing.
  • Activity and injury increase with temperature

Bipyridiliums: paraquat (Gramoxone Max, Gramoxone Extra, Gramoxone Inteon) and diquat (Reward).

Characteristics of activity:

  • Nonselective
  • Very little translocation.

Symptoms of injury:

  • Water-soaked look early.
  • Chlorosis/necrosis of plant tissue.
  • Drift injury occurs as specklin


Filed under problems: Herbicide Injury

More information is available on Hort Answers.