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Multiple Herbicide-Resistant Palmer Amaranth Confirmed in Missouri

PPO inhibitor resistance joins that of glyphosate in Missouri for Palmer amaranth.

Last September, Southern Illinois University weed scientist Karla Gage and Ronald Krausz, researcher and farm manager of SIU’s Belleville Research Center, identified Palmer amaranth with glyphosate (group 9) and PPO  (group 14) resistance north of St. Louis. Discovered in the Mississippi River bottoms, it is Missouri’s first-known instance of Palmer amaranth that resists postemergence applications of group 14 herbicides (PPO inhibitors) like fomesafen (Flexstar, Marvel, etc.), lactofen (Cobra), or aciflurofen (Ultra Blazer).

The team sent tissue to the Illinois Plant Clinic, where the samples were confirmed to have elevated copy numbers of the EPSPS gene, which leads to glyphosate resistance. The sample also tested positive for a point mutation in the PPO gene known to confer resistance to PPO-inhibitor herbicides.

PPO-inhibitor resistance in Palmer amaranth was first discovered in Tennessee in 2015. More recently, other populations have been discovered in southern Illinois. Midwest growers have had to contend with PPO-resistant populations of waterhemp for many years. Especially in Missouri and Illinois, multiple-resistant waterhemp is now the rule rather than the exception.

This new finding with Palmer amaranth reemphasizes the need for an integrated approach to the management of troublesome pigweed species like Palmer amaranth and waterhemp. This approach includes multiple herbicide modes of action and cultural practices that minimize the deposition of weed seed back into the soil, the researchers say.

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