Abstract

This is the first confirmation of 4-hydroxyphenylpyruvate dioxygenase-(HPPD)-inhibitor resistant waterhemp and the first report of 5-way multiple-herbicide-resistant (MHR) waterhemp in Ontario to Group 2, 5, 9, 14, and 27 herbicides. Seed was collected across southern Ontario in 2021 and tested for resistance to imazethapyr, atrazine, metribuzin, glyphosate, lactofen, mesotrione, and mesotrione + atrazine representing Group 2, 5, 5, 9, 14, 27, and 27 + 5 herbicides, respectively. All of the waterhemp populations collected in 2021 had 4-way resistance to imazethapyr, atrazine, glyphosate, and mesotrione. In seven Ontario counties, 5-way resistant waterhemp was confirmed which includes resistance to the HPPD-inhibiting herbicides.

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