Herbicides that inhibit the protoporhyrinogen oxidase enzyme in target plants are are in herbicide Group 14 and are collectively referred to as PPO herbicides. Commonly used PPO herbicides in Mississippi include Aim, Cobra, fomesafen (Reflex, Flexstar, various generics), Sharpen, sulfentrazone (one of the active ingredients in Sonic and the Authority line of herbicides), Ultra Blazer, and Valor SX. The PPO herbicide family contains some active ingredients that exhibit postemergence activity (Ultra Blazer, Aim, Cobra), others that exhibit residual activity (Valor SX, sulfentrazone), and still others that offer postemergence and residual activity (fomesafen, Sharpen).

The PPO herbicides have been one of the primary herbicide chemistries used for control of glyphosate-resistant Palmer amaranth in Mississippi over the last five to eight years. Because of the widespread problem with glyphosate-resistant Palmer amaranth and because there are few other options, PPO herbicides have often been used in the same field in successive years. Therefore, similar to the evolution of glyphosate resistance in Palmer amaranth, it was only a matter of time before repeated use of the same herbicide mode of action selected for Palmer amaranth plants resistant to PPO herbicides.

Poor herbicide performance has been common in parts of Mississippi over the past three years due to poor environmental conditions during the spring. Complaints of efficacy with PPO herbicides have increased in Mississippi in recent years; however, no resistance was documented prior to 2015. The first reported failure of a PPO herbicide in Mississippi in 2015 occurred in June. Samples were collected from the field and transferred to the greenhouse facility at the Delta Research and Extension Center in Stoneville. On July 22, confirmation of PPO-resistant Palmer amaranth was reported in Arkansas and Tennessee.  That resistance was confirmed using genetic testing in a lab at the University of Illinois. Subsequently, tissue samples collected from multiple counties in the Mississippi Delta were sent to the same lab for testing; however, all samples collected in Mississippi tested negative for the presence of the PPO resistance gene.

In September, 100 Palmer amaranth seed samples were collected at random from fields in 10 counties in the Mississippi Delta. These samples were subjected to traditional herbicide response screening by Dr. Vijay Nandula with the USDA-ARS in Stoneville. Seed collected from individual female Palmer amaranth plants were grown in trays and treated with fomesafen at 0.375 pounds active ingredient per acre (equivalent to Reflex at 1.5 pints per acre) in a water volume of 20 gallons per acre when plants reached 2 to 3 inches in height. Results of the initial screening are preliminary and must be repeated before official confirmation, but they indicate that Palmer amaranth resistant to PPO herbicides is likely in Mississippi.

Among the 100 Palmer amaranth samples tested, 38 had plants that survived the herbicide application at 21 days after application. The percent survival among treated plants ranged from 6 to 67%.  Samples collected in Bolivar, Coahoma, Sunflower, and Tunica counties were the most suspect in their response to fomesafen.  Subsequent research will be conducted to learn more about how the Palmer amaranth plants respond to postemergence and residual applications of PPO herbicides. Fields located in Bolivar, Coahoma, Sunflower, and Tunica counties or in counties adjacent to these could likely contain PPO-resistant Palmer amaranth in 2016.  Please contact me if you suspect a problem.