Herbicide Resistance in Peanuts Peanut Notes No. 55 2019
One has to assume that most fields with Palmer amaranth have ALS resistance (Cadre/Impose, Strongarm, and Pursuit). Cadre/Impose will not control 100% of the population but could control between 0 and 95% (probably 40 to 80% of individuals in the field.) One also has to assume the same relative to glyphosate.
ALS and glyphosate resistance is now widespread across the state. How about PPO resistance in Palmer amaranth? To date resistance to the PPO inhibitors has not been confirmed in Palmer amaranth. Dr. Everman and his colleagues have confirmed PPO resistant common ragweed in one area of the northeastern part of the state but that is the extent right now. But, may fields now have glyphosate and ALS resistant common ragweed, and this makes the PPO inhibitors extremely important in peanuts. If you see Palmer amaranth and/or common ragweed slip through Valor SX and postemergence sprays of PPO inhibitors (Cobra, Ultra Blazer, and/or Storm,) do not let those plants seed out! Let us know if you run into this situation. The PPOs are extremely important for all of our major crops. We need to protect them by reducing selection pressure and preventing suspected resistant individuals from reproducing. For this reason, Gramoxone is a critical component of early season weed management, and including residual herbicides such as Zidua, Dual Magnum, Warrant and Outlook is important because they reduce selection for resistance to PPOs.
With that said, preliminary reports across the mid-South suggest that biotypes resistant to the mode of action for Zidua, Dual Magnum, Warrant and Outlook may be present.