Weed scientists at Southern Illinois University Carbondale recently announced confirmation of protoporphyrinogen oxidase (PPO) inhibitor resistance in Palmer amaranth. Illinois has become the third state behind Arkansas (2011) and Tennessee (2015) to confirm PPO inhibitor resistance.
The SIU scientists in collaboration with weed scientists from the University of Illinois found three populations that showed control failure after treatment with PPO inhibitor active ingredients fomesafen or lactofen (Flexstar, Cobra). The study confirmed two-way herbicide resistance (PPO inhibitor and glyphosate) for weed populations in Cahokia and Collinsville, Ill. However, tests of several individual plants from one field allowed researchers to confirm the frequency of PPO resistance is at less than 20 percent of the population.
The discovery of PPO-resistant Palmer amaranth in Illinois is not surprising, said SIU scientist Karla Gage. She said the combination of the common use of PPO herbicides, the rapid evolution of Palmer amaranth, and the known long-distance dispersal of Palmer amaranth seeds with the movement of birds and machinery, caused her and other researchers to anticipate the PPO-resistance.
As for treating non-resistant Palmer amarath, Iowa State weed scientist Bob Hartzler said you already know the ropes. “The one thing we have going for us is that every corn and soybean field has waterhemp, so farmers have developed weed management programs targeting waterhemp. Programs that are effective on waterhemp should provide effective control of Palmer amaranth. That alone will make it hard for the weed to spread rapidly. However, Palmer amaranth is more aggressive and grows more rapidly than waterhemp. That reduces the window of opportunity to implement control tactics,” he said.
A lot of waterhemp is already resistant to glyphosate (Roundup), and it is likely Palmer amaranth will carry that same resistance. Farmers need to develop diversified weed management programs that use multiple herbicide sites of action and include alternative management strategies to delay further selection of herbicide-resistant weeds.”
In Illinois, one field with confirmed resistance is located less than a mile from a Palmer population that is thought to have been introduced by geese. The grower maintained the field in the Conservation Reserve Program (CRP) for more than a decade and often saw geese foraging throughout the field. Once the field was tilled and planted into soybeans, a sea of Palmer amaranth emerged. Gage said it is likely that the removal of the competing CRP vegetation allowed the dormant and newly deposited Palmer amaranth seeds to emerge from the seed bank. University of Missouri research shows that waterfowl, specifically ducks, can disperse Palmer amaranth seeds about 1,700 miles.
Considering the confirmation of PPO-resistant Palmer amaranth in Illinois, how quickly the species evolves and how far the resistant seeds may travel, growers should design robust field management programs and assume that low-level resistance is already present. A robust program includes a diversity of herbicide modes of action within and between years, along with crop rotation and correct herbicide application timing.