Palmer amaranth (Amaranthus palmeri) and waterhemp (Amaranthus tuberculatus) are the two most troublesome pigweeds in crop production systems in the United States. These pigweeds just started to appear in the Pacific Northwest (PNW). A coordinated extension and outreach effort among land-grant universities (University of Idaho and Oregon State University), Amalgamated Sugar, other commodity commissions, and industry was launched to track Palmer amaranth and waterhemp in the PNW. In 2023, tissue samples were collected from pigweeds suspected to be Palmer amaranth and waterhemp and sent to Colorado State University for KASP genotyping test to confirm if the species were Palmer amaranth and waterhemp. The KASP test confirmed that the suspected pigweeds were Palmer amaranth and waterhemp. Since the majority of these pigweeds survived multiple applications of glyphosate, 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene duplication analysis was conducted to confirm possible glyphosate resistance in the Palmer amaranth and waterhemp populations. About 70% (17 out of 23) of the Palmer amaranth tissue samples showed gene duplication of up to 184 EPSPS gene copies, indicative of glyphosate resistance. All three populations of waterhemp showed gene duplication of 5.7 to 9.2 EPSPS gene copies indicative of glyphosate resistance. The widespread glyphosate resistance in the samples collected suggests that Palmer amaranth and waterhemp being introduced into the PNW are coming from States where these weeds have developed resistance to multiple herbicide groups. This would have huge implications for weed control in vegetables and other crops in the PNW.
