Mirror, mirror, on the wall, who has the ugliest pigweed of all? The answer might surprise you.
Scientists have recently uncovered the country’s first four-way resistant Palmer amaranth populations in Kansas. So far, only in the land of Toto and Dorothy can a single pigweed population survive glyphosate, ALS herbicides, HPPD herbicides and triazines (atrazine). The Kansas State research team, led by weed scientist Vipan Kumar, are now testing this same strain of pigweed for resistance to PPO-inhibitors and auxinic herbicides (dicamba and 2,4-D), as well.
In this series, The State of the Pigweed, we checked in with 28 states to see where they stand with this weed. In this second of three stories, we explore the experience of 12 Midwest, Mid-Atlantic and Southwest states, which have become herbicide-resistant pigweed’s “new frontier.”
Although they don’t quite have the long history and experience of their battle-hardened Southern neighbors, this group of states are staring down Palmer amaranth populations with some of the most challenging herbicide-resistant traits in the country.
Meet the States
These states form a thick belt encircling the South, where herbicide-resistant Palmer amaranth got its start: Texas, Oklahoma, Kansas, Nebraska, Missouri (minus the Bootheel of southeast Missouri), Illinois, Indiana, West Virginia, Virginia, Maryland, Delaware and Pennsylvania.
For some of these states, Palmer amaranth is a native weed, or at least a long-present one. But when glyphosate-resistant species began to show up, starting between 2008 and 2014, the game changed.
“It has been a ‘common’ weed for probably 50-plus years, but growers were successful at controlling it,” explained Peter Dotray, a weed scientist who works for both Texas A&M and Texas Tech University. “Recently, in the last five years, it is now considered a ‘troublesome’ weed because of glyphosate resistance.”
For many of these states, weeds like waterhemp and marestail still dominate control strategies, but that’s starting to change. Pennsylvania, for example, first encountered Palmer amaranth in 2012. Now, six years later, standard herbicide control programs remain in place in much of the state — except for the 25 counties with Palmer amaranth, where that weed drives herbicide choice and strategies, said Penn State University Extension weed scientist Dwight Lingenfelter.
“For now, waterhemp is bigger in terms of acreage and chemicals lost to resistance,” added University of Illinois Extension weed scientist Aaron Hager. “But there’s no reason to assume Palmer will be substantially different. I think it will be similar to waterhemp, in that it will evolve resistance to multiple herbicides within individual populations.”
Resistance Thrives on the New Frontier
Most Palmer amaranth populations in these states are resistant to glyphosate and ALS herbicides, but some populations already contain the industry’s most challenging herbicide-resistant traits as well.
Illinois was among the first states in the country to confirm three-way resistant populations that weren’t controlled by PPO-inhibitor herbicides — the backbone of many soybean herbicide programs.
Missouri now also has PPO resistance, as does Indiana. The latter has also confirmed Palmer amaranth resistance to triazines (atrazine), joining Texas, Kansas and Nebraska. Now Purdue University Extension weed scientist Bill Johnson suspects another class of herbicides, HPPD inhibitors, may soon be compromised in Indiana, too.
Kansas State University weed scientist Vipan Kumar confirmed HPPD resistance in the four-way resistant population he found in Kansas in 2018. Nebraska also has pockets of resistance to HPPD herbicides in a south-central county, said University of Nebraska Extension weed scientist Amit Jhala.
In Texas, state surveys have found that some Palmer amaranth populations were “less sensitive” to recommended label rates of tembotrione (an HPPD inhibitor) and dicamba (an auxinic herbicide), Texas A&M weed scientist Muthu Bagavathiannan said.
University of Maryland plant biologist Wendy Peer is also worried about the development of auxin resistance in Maryland fields, based on field observations and some preliminary laboratory research. Her co-workers have observed Palmer amaranth surviving dicamba applications in one Maryland field, and she has observed up to 50% of pigweed populations surviving 2,4-D applications in the lab and other fields.
Given the addition of dicamba-tolerant Xtend crops and 2,4-D-tolerant Enlist crops to the landscape, many states are bracing for the arrival of auxin-resistant Palmer amaranth.
Dotray said Xtend cotton technology helped growers clean up problem fields in Texas last year, but he is urging growers not to forget the importance of pre-plant and pre-emergence residual herbicide use.
“The use of dicamba (Xtendimax, FeXapan, Engenia) alone in Xtendflex cotton or 2,4-D choline (Enlist Duo and Enlist One) alone in Enlist cotton is not effective nor sustainable” for providing season-long weed control, he said.
Maryland’s Peer shares his concerns, especially since auxinic herbicides have already shown weakness against Palmer amaranth in the state. “The chemistry I fear for the most is the combination of stacked herbicides, like glyphosate and 2,4-D and glyphosate and dicamba,” Peer said. “I believe that this combination will quickly select for resistance to these two herbicides, which may already be present in the state.”
Kansas State University’s Kumar believes the future in the fight against Palmer amaranth will not emerge from a laboratory or jug.
“We’re looking at some alternative strategies to control multi-resistant Palmer amaranth,” he said. “For example, how we can integrate cover crops with herbicide programs?”
See the first story in this series, The State of the Pigweed, here: https://www.dtnpf.com/…
For a map showing the full distribution of Palmer amaranth in the U.S., see this link from University of Wyoming weed scientist Andrew Kniss: https://plantoutofplace.com/…
Emily Unglesbee can be reached at Emily.firstname.lastname@example.org
Follow her on Twitter @Emily_Unglesbee
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