Published online by Cambridge University Press: 20 January 2017
Rigid ryegrass, an important annual weed species in cropping regions ofsouthern Australia, has evolved resistance to 11 major groups of herbicides.Dose–response studies were conducted to determine response of threeclethodim-resistant populations and one clethodim-susceptible population ofrigid ryegrass to three different frost treatments (−2 C).Clethodim-resistant and -susceptible plants were exposed to frost in a frostchamber from 4:00 P.M. to 8:00 A.M. for three nights before or afterclethodim application and were compared with plants not exposed to frost. Areduction in the level of clethodim efficacy was observed in resistantpopulations when plants were exposed to frost for three nights before orafter clethodim application. In the highly resistant populations, thesurvival percentage and LD50 were higher when plants were exposedto frost before clethodim application compared with frost after clethodimapplication. However, frost treatment did not influence clethodim efficacyof the susceptible population. Sequencing of the acetyl coenzyme Acarboxylase (ACCase) gene of the three resistant populations identifiedthree known mutations at positions 1781, 2041, and 2078. However, mostindividuals in the highly resistant populations did not contain any knownmutation in ACCase, suggesting the resistance mechanism was a nontargetsite. The effect of frost on clethodim efficacy in resistant plants may bean outcome of the interaction between frost and the clethodim resistancemechanism(s) present.
Associate Editor for this paper: Franck E. Dayan, USDA-ARS.