Clemson scientist targets herbicide-resistant pigweed

When Mike Marshall was appointed as a South Carolina Extension weed scientist at the end of 2007 one challenge was made very clear — Palmer pigweed in cotton. The challenge, he admits, is a formidable one, but one that can be managed.

Marshall, a native of Kansas, earned bachelors and masters degrees in agronomy from Kansas State University. As part of his masters programs he studied the biology of herbicide resistant weeds and got a good introduction and education on the science of herbicide resistance.

After finishing his degrees at Kansas State, Marshall earned a Ph.D in weed science from the University of Kentucky. He worked primarily in Extension, participating in grower field days and meetings. During his doctoral work, Marshall worked on weeds of row crops and got a good introduction to the Extension/grower relationships.

From Kentucky, he went on to Michigan State University, where he worked for two years as post-doctoral scholar, working on ornamentals. “It was a 180 degree shift from row crops to working with Christmas trees and smaller ornamental crops,” he says. During this work, he was a part of the team that first identified glyphosate resistance on marestail in Christmas trees.

Though Christmas trees are about as far away from cotton, corn and soybeans as it gets, the introduction into glyphosate resistance played a key role in shaping at least the early part of his career. And, his work at Kansas State put him in an ideal position to put together both the basic science of resistant weed biology and the applied science of working with resistant weeds in crops.

In his new job at Clemson, where he will be working out of the Edisto Research and Education Center in Blackville, S.C., Marshall will have the opportunity to put all his skills together in what has become an all-out war between growers and the ever-increasing pressure of weed resistance to glyphosate, ALS-inhibitors and other families of herbicides and Palmer pigweed.

“Glyphosate resistance is a big challenge — it’s likely to be a problem for growers over the next few years. It’s not going away and it’s not a problem growers can solve in a year or two. On the other hand, there are some tools to manage glyphosate resistant weeds, but they have to be used carefully,” Marshall warns.

For example, he notes that growers have Reflex, a PPO inhibitor and Prowl, both of which can provide excellent control of small pigweed. The DNA family of herbicides, including Prowl, has documented resistance problems with Palmer amaranth. Other amaranth species in the U.S. have shown resistance to the PPOs, so growers have to be careful on the use of these non-resistant herbicides, he explains.

“In South Carolina, we have a monoculture of Roundup Ready cotton, corn and soybeans. Peanuts are about the only crop grown in rotation in South Carolina that is not heavily dependent on glyphosate. We have to be careful to not to over-use the PPO herbicides that work so well on Palmer pigweed in cotton,” the Clemson weed scientist stresses.

“We have some new resistance traits, like dicamba-resistant soybeans and other technologies that are in the pipeline and will be tremendous tools for growers to use in the future. Right now, we have to use the herbicides we have wisely and keep chipping away at the whole arena of weed resistance in cotton, soybeans, corn, wheat and other crops we grow in South Carolina and manage the problem until we get these new technology tools,” Marshall says.

“It’s a hard sell and hard for growers to do, but we have to move away from the glyphosate monoculture. Growers have to understand the modes of action of different families of herbicides.

“Cotton growers need to understand that herbicides used in rotation crops may have different names, but if these materials are in the same family of herbicides and have the same mode of action and are used too extensively to overcome glyphosate resistance, the grower can be increasing the risk of multiple resistance.”

In North Carolina, Alan York and his team of weed scientists has already documented ALS and glyphosate resistance in the same pigweed. Developing multiple resistance in the same pigweed is a problem growers don’t want, the famed North Carolina weed scientist repeats to growers over and over at meetings.

“It will be exciting to help growers find some different paths to go down in managing herbicide resistance. “I’m ready to get out and work with growers, having been mostly confined to the office over the winter,” Marshall says.

In the long-run South Carolina growers will benefit from a slightly different look at the whole spectrum of crop-herbicide interaction.

e-mail: [email protected]

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