A herbicide is a chemical substance that has been developed to kill plants or stop them growing. These chemicals can be selective, targeting specific types of plants or non-selective, targeting all plants.
There are three main methods that can be used to control weeds, physical, chemical and biological. Physical control entails mechanical removal by for instance tilling, or by burning or simply using one’s hands. This is not an easy option where a crop plant is also being grown.
We sent Palmer amaranth that we were confident was glyphosate/PPO resistant from north Shelby County, Tennessee to my colleague Dr. Tom Mueller in Knoxville last fall. He had a graduate student, Alinna Umphres, that subsequently screened this Palmer amaranth for its tolerance to a number of herbicides applied POST. As a check, the researchers also screened Palmer amaranth sourced from Knox County that was known to be resistant to glyphosate but not to PPO herbicides. The herbicides chosen to screen were then applied to the Palmer when it was 4” tall.
It is known that the PPO-resistance in our Shelby County population has at least 3 different genes for resist ance to PPOs. However, we are still finding Palmer amaranth that is resistant to PPO herbicides but does not contain any of those genes. A possible reason is a 4 th resistance mechanism is metabolism-based where the plant is producing enzymes that tie up the PPO herbicides. These enzymes can also tie up other herbicides as well. Last year’s field data from Arkansas and now this greenhouse data from Tennessee would suggest that metabolism could very well be an issue for at least some of our PPO-resistant Palmer amaranth in the Midsouth.