PPO-Inhibitor (Group 14) Resistant Waterhemp Confirmed in Seven Counties (04/28/22)
We conducted a survey of late-season pigweed escapes last fall to test for herbicide resistance across North Dakota. We tested over 50 pigweed samples for resistance to glyphosate (Group 9), ALS-inhibitors (Group 2), and PPO-inhibitors (Group 14). We received samples from 16 counties across the state, and samples included redroot pigweed, Powell amaranth, waterhemp, and Palmer amaranth.
After screening for resistance in the greenhouse, we worked with the National Agricultural Genotyping Center (NAGC) for genetic confirmation of resistance, where possible. One test the NAGC utilizes is a rapid assay to detect a mutation that confers resistance to PPO-inhibiting herbicides in waterhemp. Using this test, we confirmed PPO-resistant waterhemp in 7 counties across North Dakota (Cass, Grand Forks, Griggs, Logan, Ramsey, Richland, Stutsman). All populations with resistance to PPO-inhibitors also had resistance to glyphosate and ALS-inhibitors.
So, what are some management considerations for PPO-inhibiting resistant waterhemp? The important point to know is that this mutation confers resistance to the postemergence PPO-inhibitors (fomesafen, lactofen, acifluorfen). The soil applied PPO-inhibitors (flumioxazin, sulfentrazone) still provide some control, but expected length of residual control will be reduced (approximately half the expected residual compared to a sensitive population). Three-way resistance (glyphosate, ALS-inhibitors, and PPO-inhibitors) in waterhemp limits the effective postemergence options to glufosinate, dicamba, and 2,4-D in soybean that have appropriate resistance traits. Our standard recommendation of overlapping residual Group 15 herbicides becomes more important in these populations to reduce the number of waterhemp plants that we are trying to control with postemergence herbicides. The NAGC will offer commercial testing for PPO-inhibitor resistance in waterhemp this growing season. Instructions on how to submit samples will included in the next Crop and Pest Report.
Extension Weed Specialist
