Confirmation and Management of Atrazine-Resistant Palmer Amaranth ( <i>Amaranthus palmeri</i> ) in North Carolina

Abstract North Carolina farmers have extensively relied on atrazine for effective weed control in corn, particularly for Palmer amaranth. In 2016, a grower from Washington County, NC, reported a control failure of Palmer amaranth in a corn field treated with atrazine. Greenhouse studies were conducted with the objectives to confirm if the Washington County Palmer amaranth population was resistant to atrazine and to determine other effective postemergence herbicides. Two atrazine-susceptible Palmer amaranth populations collected from Edgecombe and Johnston County were used for comparison. Greenhouse response assays confirmed the Washington County population is resistant, with a lethal dose causing 50% mortality (LD 50 ) of 1,253 g ai ha -1 . This population was 7- and 9-fold less susceptible to atrazine compared with the Edgecombe (LD 50 : 208 g ai ha -1 ) and Johnston County (LD 50 : 155 g ai ha -1 ) populations, respectively. Additionally, the LD 80 (lethal dose causing 80% mortality) of the Washington County population (12,107 ga ai ha -1 ) was more than five times greater than the maximum labeled rate for atrazine. A subsequent herbicide screen that utilized field labeled rates suggested that 2,4-D, dicamba, fomesafen, glufosinate, and mesotrione provide at least 79% or greater control of the Washington County population. The Washington County population was not effectively controlled by thifensulfuron or glyphosate, both of which have widespread resistance documented throughout North Carolina. The results of these experiments provide evidence that the Washington County population is resistant to atrazine, glyphosate, and thifensulfuron, and effective herbicides to manage this population were confirmed.