A novel pattern of herbicide cross-resistance in a trifluralin-resistant biotype of green foxtail [Setaria viridis (L.) beauv.]

Abstract A trifluralin-resistant (R) biotype of Setaria viridis is found in areas of Manitoba, Canada where trifluralin is utilized as the principal or sole herbicide. In this study, we examine the crossresistance pattern of this biotype to other mitotic disrupter herbicides utilizing growth measurements and electron microscopy to monitor the resistance. Compared to a trifluralin-susceptible (S) biotype, the R biotype is cross-resistant to all dinitroaniline herbicides tested, with I 50 R S ratios [the concentration of herbicide required to inhibit root growth of the R biotype by 50% divided by the concentration which induces the same effect for the S biotype] ranging from 1.6 to 14.8. This R biotype is also cross-resistant at about the same level to amiprophosmethyl and dithiopyr, but exhibits no resistance to pronamide, sindone B, barban, or the microtubule stabilizer taxol. The highest level of resistance is to the structurally unrelated herbicides DCPA (I 50 R S > 50 ) and terbutol (I 50 R S = 13.4 ). This is the first reported incidence of resistance to these two herbicides. Ultrastructural observations show herbicide-induced abnormalities are either reduced or absent in the R biotype. The S biotype is actually less susceptible to chlorpropham and propham ( R S I 50 = 0.7) than the R biotype. The high level of resistance to the phragmoplast-disrupting microtubule herbicide DCPA, as well as terbutol, and a lower level of resistance to a number of other tubulin-interacting microtubule disrupters, indicate that the R biotype may contain an alteration in a cytoskeletal protein that stabilizes phragmoplast microtubule arrays.