The University of Illinois Plant Clinic has been diagnosing plant problems since 1976, but the array of services provided has grown and become increasingly sophisticated in recent years. Two years ago, the Plant Clinic began using molecular protocols to test for herbicide resistance to glyphosate and PPO inhibitors in waterhemp. Last year, they expanded the service to Palmer amaranth and recently added a method for distinguishing waterhemp and Palmer using molecular methods. In a U of I Bulletin post, the Plant Clinic published a report of their findings for the 2016 season.
"Almost twice as many whole fields were tested in 2016 compared to 2015: 593 vs. 338," says U of I Extension assistant dean for agriculture and natural resources and Plant Clinic director Suzanne Bissonnette.
The clinic received samples from 10 states across the Midwest in 2016, with the majority of samples coming from Illinois. In the 378 samples from Illinois, 48 percent were resistant to both glyphosate and PPO inhibitors. Resistance to both herbicides was detected in 82 percent of samples from Missouri, but only 11 samples were tested from that state.
"Fields with plants that are positive for both glyphosate and PPO inhibitor resistance are of particular concern, due to the limited possibilities for control of these weeds," says plant diagnostic outreach Extension specialist Diane Plewa.
Palmer amaranth in Illinois was not known to be resistant to PPO inhibitors, but that is no longer the case according to the Plant Clinic results."Several samples from southwestern Illinois were confirmed to be PPO inhibitor-resistant (three from Madison, and one from St. Clair counties) in our testing," Plewa reports.
The tests detect the most common mechanism of resistance to the two chemicals: target-site mutations. However, waterhemp and Palmer amaranth are known to use metabolic pathways to detoxify herbicides in other classes. Therefore, even though more than half of the fields sampled in Illinois did not show resistance to glyphosate and PPO inhibitors, farmers should not assume these weeds can be killed by alternative herbicides.