PPO inhibitor resistance confirmed in Palmer amaranth

Southern Illinois University Carbondale, in coordination with testing by the University of Illinois, recently confirmed protoporphyrinogen oxidase (PPO) inhibitor resistance in Palmer amaranth (Amaranthus palmeri). Illinois has become the third state behind Arkansas (2011) and Tennessee (2015) to confirm PPO inhibitor resistance.

Three populations showed control failure after treatment with PPO inhibitor active ingredients fomesafen or lactofen (Flexstar, Cobra). SIU researchers collected tissue samples and submitted them to the U of I for DNA analysis. The analysis confirmed two-way herbicide resistance (PPO inhibitor and glyphosate) for Cahokia and Collinsville, Ill., populations. 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.

Positive test results also showed that plants were still heterozygous for the mutated gene conferring resistance -- meaning the plants still have one good, un-mutated copy of the gene. This suggests that these sites are still in the early stages of resistance development, compared to PPO-resistant waterhemp populations, where homozygous resistance (two copies of the mutated gene) is common.

The discovery of PPO-resistant Palmer amaranth in Illinois is not surprising, Karla Gage, weed scientist and assistant professor of plant, soil and agricultural systems, and plant biology, said. She said the combination of common use of PPO herbicides, the rapid evolution of Palmer amaranth to evolve resistance to multiple herbicide modes of action, 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.

Palmer amaranth is a dioecious plant, meaning there are males and females in each population. Cross-pollination must occur between plants in order to produce seeds. This gives Palmer amaranth an advantage: when a high rate of genetic recombination occurs and an individual plant produces half a million seeds each year, the odds are favorable for a few of the offspring to possess the ability to survive a herbicide application. Plants also have a growth rate of two to three inches per day, so plants may quickly outgrow the size limits for herbicide application, stated on the herbicide label.

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 where none was found before. 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 seedbank. University of Missouri research has shown 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.

The 2017 growing season will likely see growers switching from Roundup Ready to other crop technologies, such as the Liberty Link system. Proper stewardship of these technologies is essential. Waiting too late to apply herbicides and spraying weed populations that are too tall to control will quickly result in resistance to additional herbicides by selecting for survival of the most tolerant plants.