Herbicide Injury Symptomology: Photosynthesis Inhibitor Injury

Triazines, Phenylureas, Benzothiadiazoles, Nitriles, and Pyradazines

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Triazines

smSencorc (10435 bytes) smAtrazs (19619 bytes) smTriazines4 (12532 bytes) smTriazines1 (7150 bytes)

Trade Name and Uses:

  • Aatrex (Atrazine) 
    -corn, grain sorghum
  • Princep (Simazine)
    -corn
  • Bladex (Cyanazine)
    -corn
  • Sencor/Lexone (Metribuzin)
    -soybeans, corn

Mode of Action: Binds to the D-1 quinone protein, which is essential for photosynthesis.

Injury Symptoms: Have soil and foliar activity, which can lead to chlorosis and necrosis at leaf tips and margins on older leaves (lower leaves) first, followed by interveinal chlorosis and lower leaf drop. Photosynthesis inhibitors do not prevent seedlings from germinating or emerging. Injury symptoms only occur after the cotyledons and first leaves emerge. In broadleaf plants, yellowing between leaf veins may occur. Older and larger leaves will be affected first because they take up more of the herbicide from the water solution and they are the primary photosynthetic tissue of the plant. Injured leaf tissue will eventually turn brown and die. Due to the chemical nature of the herbicide-soil relationship, injury symptoms are likely to increase as soil pH increases (above 7.2).

Phenylureas

smLoroxc2 (11854 bytes)

Trade Name and Uses:

  • Lorox (Linuron)
    -corn, soybeans
  • Spike (Tebuthiuron)
    -pastures, noncropland

Mode of Action: Binds to the D-1 quinone protein, which is essential for photosynthesis.

Injury Symptoms: Soil and foliar activity - very similar to triazine.

Benzothiadiazoles

smBasags (16170 bytes) smBenzothiadiazoles1(6209 bytes) smBenzothiadiazoles3 (5552 bytes) smBenzothiadiazoles2 (9308 bytes)

Trade Name and Uses:

  • Basagran (Bentazon)
    -corn, soybeans, grain sorghum

Mode of Action: Binds to the D-1 quinone protein, which is essential for photosynthesis.

Injury Symptoms: Has foliar activity, so plant injury is confined to foliage that has come into contact with the herbicide. Affected leaves will become yellow or bronze in color and eventually turn brown and die. Injury symptoms can look similar to cell membrane disruptors. COC and other additives may increase weed control and crop injury symptoms. Hot, humid weather will also increase likelyhood of injury.

Nitriles

smBuctrilc (16282 bytes) smNitriles2 (6300 bytes) smNitriles1 (5395 bytes) smBuctrils (13039 bytes)

Trade Name and Uses:

  • Buctril (Bromozynil)
    -corn, alfalfa, small grains

Mode of Action: Binds to the D-1 quinone protein, which is essential for photosynthesis.

Injury Symptoms: Plant injury is confined to foliage that has come into contact with the herbicide. Foliage that has been thoroughly convered with the herbicide will turn yellow, then brown, and die. Contact of a low rate of herbicide with leaves may result in spotting or speckling of the leaf surface. COC or other additives may intensify injury symptoms. On grass plants, corn, or grain sorghum, leaf edges become chlorotic and necrotic.

Pyradazines

Trade Names and Uses:

  • Tough (Pyridate)
    -corn

Mode of Action: Binds to the D-1 quinone protein, which is essential for photosynthesis.

Injury Symptoms: Plant injury is confined to foliage that has come into contact with the herbicide. Foliage that has been thoroughly convered with the herbicide will turn yellow, then brown, and die. Contact of a low rate of herbicide with leaves may result in spotting or speckling of the leaf surface. COC or other additives may intensify injury symptoms.


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