Blackgrass (Alopecurus myosuroides Huds.) Multiple Resistance to ACCase- and ALS-Inhibitors and Its Competition with Winter Wheat

<b>Background/Objectives:</b> The herbicide resistance of blackgrass (<i>Alopecurus myosuroides</i> Huds.) is one of the most serious problems in the winter cereal monoculture in Europe. Recently, Greek farmers expressed complaints of reduced susceptibility of this weed to winter wheat herbicides. Keeping this in mind, this study focused on the investigation of blackgrass resistance to herbicides at both phenotypic and molecular levels. <b>Methods</b>: Whole-plant rate-response pot assays were conducted to study the possible evolution of resistance (cross- or multiple-resistance) in a blackgrass population to ACCase- and ALS-inhibiting herbicides. Analysis of the <i>ACCase</i> gene sequence, herbicide metabolism study and competition with winter wheat studies were also conducted. <b>Results</b>: High levels of cross-resistance mainly to the ACCase post-emergence clodinafop-propargyl, medium to fenoxaprop-P-ethyl, cycloxydim, pinoxaden, as well as lower levels of resistance to ALS-inhibitors (mesosulfuron-methyl + iodosulfuron-methyl-sodium and pyroxsulam) were confirmed. In addition, the pre-emergence soil-applied herbicides chlorotoluron + diflufenican and prosulfocarb provided excellent control of the S and R blackgrass populations. The analysis of the ACCase gene sequence revealed a point mutation at position 1781, resulting in an amino acid substitution from isoleucine (Ile) to leucine (Leu). Furthermore, the combined application of the herbicides with piperonyl butoxide (PBO, applied 2 h before herbicide application) indicated that there was herbicide metabolism, which may be mediated by cytochrome P450. The R blackgrass population, when grown in competitive interaction with winter wheat, produced more tillers and aboveground fresh weight compared to the S population and caused greater reduction in winter wheat. <b>Conclusions</b>: The results suggest that a blackgrass population has developed multiple resistance to ACCase- and ALS-inhibiting herbicides, due to <i>ACCase</i> gene mutation and herbicide metabolism. No fitness cost and no compromised competitive ability associated with the blackgrass resistance were observed.