Confirmation of glyphosate-resistant waterhemp (Amaranthus tuberculatus) in New York

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Abstract
Waterhemp has become a serious management challenge for field crop growers in New York. Two putative glyphosate-resistant (GR) waterhemp populations (NY1 and NY2) were collected in 2023 from two soybean fields in Seneca County, NY. The objectives of this research were to 1) confirm and characterize the level of glyphosate resistance in waterhemp populations from New York relative to a known glyphosate-susceptible population from Nebraska (NE_SUS), and 2) evaluate the efficacy of various postemergence herbicides for GR waterhemp control. Based on the shoot dry weight reductions (GR50 values) in a dose-response study, the NY1 and NY2 populations exhibited 5.6- to 8.3-fold resistance to glyphosate compared with the NE_SUS population. In a separate study, postemergence herbicides such as dicamba, glufosinate, lactofen, and 2,4-D applied alone or in a mixture with glyphosate or glufosinate had provided 89% to 99% control and ≥97% shoot dry weight reduction of NY1 and NY2 populations 21 d after treatment. Greater than 98% control of the NE_SUS population was achieved with tested postemergence herbicides, except mesotrione (62% control). Furthermore, atrazine, chlorimuron + thifensulfuron, and mesotrione were the least effective in controlling NY1 and NY2 populations (42% to 59% control and 50% to 67% shoot dry weight reductions, respectively). These results confirm the first report of GR waterhemp in New York. Growers should adopt effective alternative postemergence herbicides tested in this study to manage GR waterhemp.

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