Alabama: PPO Resistant Pigweed Likely Present in Northern Areas

    Timely recommended applications of Flexstar GT (fomesafen + glyphosate) did not control Palmer amaranth in a closely monitored field in North West Alabama (Lauderdale County).  A herbicide strip trial for Palmer amaranth control was embedded in this field. Two sequential applications of Flexstar GT failed to control many Palmer amaranth plants 2″ or less.  Samples were taken and sent off for molecular testing.

    These samples came back as susceptible to PPO inhibiting herbicides however, the molecular test only tests for resistance to one site of action.  Upon further discussion with other colleagues and those that do the molecular testing, it appears that other issues or resistance mechanisms could be occurring that would not be confirmed in this testing.

    Given the amounts of PPO herbicides applied to these small Palmer plants, resulting symptomology, and based on conversations with other weed scientists, these should be treated as PPO resistant until proven otherwise.

    Current efforts are in place to begin seed collection and screening processes by growing out plants for two seasons to ensure that the progeny are resistant in the same way as are the parent plants. This is time consuming but is necessary to confirm what we see in the field.  However, given the facts in the first paragraph, extreme caution should be used in this instance since strong rates of PPO herbicide did not work on 2″ or less Palmer.

    PPO Resistant Palmer pigweed began showing up in the mid-south a few years ago. PPO herbicides are Group 14 herbicides. They include diphenlethers (acifluorfen, fomesafen, lactofen, oxyfluorfen), N-phenylphthalimide (flumichlorac, flumioxazin), Oxadiazole (oxadiazon),  Pyrimidinediones (saflufenacil) and Aryl triazonones (sulfentrazone, carfentrazone-ethyl, fluthiacet-ethyl)

    Herbicide Resistance Info

    They are commonly used in soybean, both PRE and POST, and cotton (burndown and POST), and vegetable crops. PPO herbicides used in this area commonly include products like Reflex, Prefix, Sharpen, Valor, Cobra, and Blazer.

    PPO resistance is a huge concern, especially in soybean, because many of the herbicides that are utilized in soybean production are PPO herbicides.  Without the use of PPO herbicides, we will be limited to using very few POST applied herbicides. Most of the time when plants are resistant to PPO herbicides they are typically resistant to glyphosate and ALS herbicides as well.

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