VII. Pigment inhibitors (SOA 13, S0A 27)
The pigment inhibitors include herbicide families that interfere with pigment production and protection of chlorophyll. They include:
- Isoxazolidinone.
- Isoxazole.
- Triketone
- Pyrazolone
Carotenoid pigments protect chlorophyll from decomposition by sunlight. As a result, the affected plant parts become white to translucent and appear bleached. Pigment inhibitors have pre-emergence and postemergence activity. Applied pre-emergence, they are taken up by plant roots and shoots and move in the xylem to the plant leaves. Susceptible weeds will emerge as white plants before dying.
Applied postemergence, they are absorbed through the foliage and move acropetally (upward) to leaf margins. They are most effective on small weeds. Off-target movement to nontarget plants also may cause foliage to turn white. The fate of the plant is dependent on species and size of the plant; small plants are more susceptible than large plants.
Phenotype from pigment inhibitors could be confused with chimera, or a plant part that is a mixture of two or more genetically diverse types of cells. Chimeras also may arise by a mutation in cells of a growing region. The new kind of tissue may be conspicuously different from old (as when it is bleached instead of green) (Photo 51). More common, the difference is evident only on special investigation, as when the number of chromosomes is altered.
Although injury symptoms are similar, specific site of action is different between herbicide families. Isoxazole, triketon and pyrazolone family herbicides are soluble and are a persistent herbicidally active metabolite in soil. Thus, these families are restricted on course-textured soils or soils with a shallow water table.
Herbicide use
Clomazone (Command) for soybean.
Injury symptoms
All or portions of the true leaves will turn white. Sugarbeet is relatively tolerant to clomazone residual in soil or clomazone volatility and generally plants with symptoms (Photo 52) will live, turn green and produce a nearly normal-size root at harvest.
Site of action
1-deoxy-D-xyulose 5-phosphate synthase (DOXP synthase).
Herbicide use
Isoxaflutole (Balance Flexx) for corn.
Injury symptoms
Isoxaflutole-family herbicides will turn sugarbeet leaves white following spray drift. Sugarbeet injury severity is dependent on dose; injury ranges from mottling injury of older tissue, bleaching of newest leaves, or bleaching and death of sugar beet.
Site of action
4-hydroxyphenyl-pyruvatedioxygenase (HPPD). Isoxazole and triketone family herbicides have the same mode of action; inhibition of carotenoid biosynthesis.
Herbicide use
- Mesotrione (Callisto) for corn, flax and oat. A component of many premixes including with Resicore.
- Tembotrione (Laudis) for corn. Component of Diflexx DUO for corn and Capreno for corn.
- Bicyclopyrone for barley, corn and wheat (component of Acuron for corn and component of Tallinor for wheat and barley).
Injury symptoms
Triketone family herbicides will turn sugarbeet leaves white (Photos 53, 54, 55) because of spray drift (mesotrione) or sugarbeet will emerge with bleached leaves (tembotrione).
Sugarbeet should not follow Laudis if cumulative precipitation between Laudis application and sugarbeet planting is less than 20 inches or less than a 10-month rotational interval between Laudis application and sugarbeet planting (Photo 56).
Site of action
4-hydroxyphenyl-pyruvatedioxygenase (HPPD).
Herbicide use
- Topramezone (Impact, Armezon) for corn.
- Pyrasulfotole (component of Huskie, Huskie Complete and Wolverine Advanced) for barley and wheat.
Injury symptoms
Pyrazolone family herbicides will turn sugarbeet leaves white because of spray drift (Photo 57).
Site of action
4-hydroxyphenyl-pyruvatedioxygenase (HPPD).
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Be sure that the area you wish to treat is listed on the label of the pesticide you intend to use. Remember, the label is the law.
Reviewed in 2018