Distribution of Italian ryegrass [ <i>Lolium perenne</i> L. ssp. <i>multiflorum</i> ] Resistance to Postemergence Herbicides Across North Carolina

Abstract North Carolina farmers face mounting issues with Italian ryegrass, as herbicide-resistant populations continue to be found in more fields across the state. Increased selection pressure has led to cases of multiple and cross resistance, leaving growers with fewer options for effective weed control. The objective of this study was to understand the Italian ryegrass frequency and the distribution of resistance to commonly used postemergence herbicide sites of action. Italian ryegrass seedheads were collected from around the state of North Carolina in June 2022. A statewide grid sample resulted in 115 samples collected from wheat fields, with observed Italian ryegrass escapes. Seeds were germinated in greenhouses, and herbicides were applied to 4-leaf plants. Treatments included a non-treated check, mesosulfuron, pinoxaden, glyphosate, paraquat, and glufosinate, all at 1x rates according to each label. Populations controlled ;≤ ;50% by a specific herbicide were considered putative-resistant. Ten populations were found to be putative resistant to mesosulfuron, 17 to pinoxaden, 2 to glyphosate, 1 to paraquat, and 44 to glufosinate. One four-way-resistant population was found, along with two three-way-resistant populations. The results from this collection were compared with populations from a similar statewide collection conducted in 2012. These populations were screened for response to mesosulfuron, pyroxsulam, pinoxaden, and diclofop, we also screened these populations for response to glyphosate, paraquat, and glufosinate. Out of 102 populations, none were found to be resistant to glyphosate and paraquat, while 91 populations were determined to show resistance to glufosinate. These results are helpful tools for growers and Extension personnel as they develop effective weed management programs.