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Pigment inhibitor herbicides

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.

Chimera on sugarbeet _ loss of pigment
Photo 51. Chimera in sugarbeet. Photo: Oliver Neher, Amalgamated Sugar

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.

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Reviewed in 2018

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