University of California
Herbicide Symptoms

Microtubule Assembly Inhibitors

HRAC Group: K1

WSSA Group: 3

Microtubule Inhibitors are generally applied preemergence to control annual grasses and some broadleaf weeds in many crops and turf grass. These herbicides are absorbed by both roots and shoots of emerging seedlings but are not readily translocated. The emerging shoot is the primary absorption and action site in grass species. These herbicides are mitotic poisons that inhibit cell division. Thus, the meristematic regions, such as the growing points of stems and roots, are most affected.

Benzamide, benzoic acid (DCPA), dinitroaniline, phosphoramidate, and pyridine herbicides are examples of herbicides that bind to tubulin, the major protein needed to polymerize microtubules that are essential for cell division.  Herbicide-induced microtubule loss may cause the observed swelling of root tips as cells in this region neither divide nor elongate.

Injury symptoms: Symptoms on grass species include short, swollen coleoptiles. Injured broadleaf plants often have swollen hypocotyls. Sometimes these herbicides cause callus formation and brittle stems near the soil surface, which may break over during the growing season. Both grasses and broadleaves may have short, stubby secondary roots and club-shaped roots. Consequently, the plants may be stunted and exhibit nutrient deficiency or drought symptoms because of the poorly developed root system. Seeds of treated broadleaved plants germinate, but they either fail to emerge or emerge as severely stunted seedlings.  Other injury symptoms include thickened and shortened lower stems and small and crinkled leaves.

Chemistry Group and Common Names of Microtubule Assembly Inhibitors
Used in the United States

ChemistryCommon Name
Benzamide Propyzamide (= pronamide)
Benzoic acid DCPA
Dinitroaniline Pendimethalin
  Benefin
  Ethalfluralin
  Oryzalin
  Trifluralin
  Prodiamine
Pyridine Dithiopyr
  Thiazopyr
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