Kochia, a weed that is rapidly becoming more abundant across southern Canadian prairies and the Great Plains of the United States, can reduce crop yields by up to 60 percent. Fighting this weed has become difficult because more than 90 percent of kochia populations are now resistant to acetolactate synthase (ALS)-inhibiting herbicides. The phenomenon of negative cross-resistance, however, may offer another path to defeating the spread of this weed.

The current issue of the journal Weed Technology reports on a greenhouse test of kochia plants. Six alternative herbicides were tested on kochia plants with the resistant mutation. Researchers were looking for differences in the reactions of the resistant kochia compared with the wild plant, which is still susceptible to herbicides.

When a plant becomes resistant to one herbicide, other physiological changes may occur that result in increased sensitivity to other herbicide families. The mutated, resistant plant that is more susceptible to the second herbicide is displaying the characteristic of negative cross-resistance.

By using negative cross-resistance to their advantage, weed scientists can outmaneuver the resistant plants. A plan of resistance management can be formulated to attack the weeds with different herbicides, controlling the resistant populations.

In the current study, researchers treated plants from six ALS resistant kochia accessions that have the Pro197 or Trp574 mutation with six alternative herbicides that attack different sites and growth processes of the plant. No difference was noted between the resistant and the susceptible kochia plants when they were exposed to the herbicides bromoxynil, fluroxypyr, or glyphosate. However, one accession with the Trp574 mutation did show negative cross-resistance.

When exposed to pyrasulfotole, mesotrione, and carfentrazone herbicides, ALS-resistant kochia were, 80, 60, and 50 percent more sensitive than the ALS-susceptible plants. Rather than being ALS-inhibiting, these herbicides target different functions of the plant.

Full text of the article, “Negative Cross-Resistance of Acetolactate Synthase Inhibitor–Resistant Kochia (Kochia scoparia) to Protoporphyrinogen Oxidase– and Hydroxyphenylpyruvate Dioxygenase–Inhibiting Herbicides,” Weed Technology, Vol. 26, No. 3, July-September 2012, is available at http://www.wssajournals.org/.