Timely applications of recommended rates of Flexstar did not control Palmer amaranth in two fields in West Tennessee.
Palmer amaranth samples were taken from these fields and sent to the University of Illinois to test for PPO herbicide resistance. Dr. Pat Tranel at the U of I used a molecular test that looks for the gene that confers PPO resistance in waterhemp, which is a close cousin to Palmer amaranth. One Palmer amaranth from both fields tested positive for the PPO resistance gene. After sequencing, the resistance was found to be the same type as found in PPO resistant waterhemp in the Midwest.
PPO mode of action herbicides commonly used POST in our area are Flexstar, Reflex, Ultra Blazer, Cobra, Aim, Cadet or Resource or combination products containing PPO herbicides such as Prefix. PPO herbicide resistance in waterhemp is very common in the Midwest but has never been documented in Palmer amaranth. This resistance mechanism essentially makes pigweeds less sensitive to PPO inhibiting herbicides.
On a positive note, PPO-inhibiting herbicides still provide soil applied residual control of waterhemp in the Midwest, though the length of residual is shortened compared to PPO-susceptible waterhemp. However once the pigweed emerges, they quickly become immune to PPO herbicides.
So do we have Palmer amaranth resistant to PPO herbicides in Tennessee? There is a scientific process that weed scientists go through before we declare a weed resistant to a given herbicide. It involves looking at the suspect weed in comparison to a known “non-resistant” biotype of the same weed, looking at various rates, and making sure that the resistance is heritable. The heritable part is why we grow plants out for two seasons and make sure the progeny are just as resistant as the parents. We are just starting this process. Since stout rates of some PPO herbicides have not consistently controlled 1-inch to 2-inch Palmer amaranth in isolated locations and now we have Dr. Tranel’s gene test results, one can surmise we likely have PPO-resistant Palmer in Tennessee.
What do we need to do at this point? There are still significant acres of soybean that may be planted in Tennessee. These fields need to be managed with at least two herbicides effective on Palmer amaranth applied PRE and then again if possible early POST. In other words, overlay residuals and do not allow the Palmer to emerge.
A good program would be to utilize Boundary (Dual Magnum + metribuzin) or Intimidator (Dual Magnum + metribuzin + Reflex) applied PRE followed by Zidua or Dual Magnum early POST.
Another option would be Prefix or Fierce applied PRE followed by Zidua or Dual Magnum early POST. This option is assuming that the PPOs will still provide some level of Palmer control PRE similar to waterhemp in the Midwest.
Of course Liberty is still an effective option in LL soybeans. However, it should never be applied alone but with a PRE program such as mentioned above and then applied early POST with Dual Magnum or Zidua tankmixed with it.