Research Highlights
Searching for a Palmer Amaranth Bioherbicide

By Laura Temple

Palmer amaranth continues its invasive spread through U.S. soybean fields. A member of the pigweed family, it is known for herbicide resistance – and its ability to develop resistance to new herbicides quickly.

Burt Bluhm, professor of entomology and plant pathology for the University of Arkansas, is leading the search for new, naturally occurring options to control Palmer amaranth. The Arkansas Soybean Promotion Board is funding this research.

“The checkoff invests in research to keep soybean farmers on the leading edge,” says Doug Hartz, a professional farm manager based in Stuttgart and member of the Arkansas Soybean Promotion Board. “As a board we fund practical independent research that addresses challenges we are seeing in producer’s fields to improve their operations.”

Palmer amaranth is a significant challenge, but Bluhm believes a bioherbicide could be a solution.

“Bioherbicides use a living organism, like a fungus or bacterium, or its derivatives, to control weeds,” he explains. “We are focusing on fungal pathogens to create a novel, highly aggressive bioherbicide that can control Palmer amaranth and other pigweeds. New technology allows us to dive into the genetics of pathogens to find natural enemies of these weeds and develop them into a commercial bioherbicide.”

The search began with Bluhm’s team collecting diseased pigweeds throughout Arkansas and beyond. They isolated the fungi that attacked the plants to create a library of options to screen for bioherbicide potential.

“The first phase of finding and isolating potential fungal pathogens took about a year,” he says. “Though we continue to add new pathogens to our library, we now focus on the screening and development process.”

In the lab, Bluhm’s team screens and rates the aggressiveness of these pathogens on Palmer amaranth. The focus is fungi specific to pigweed species, to take advantage of pathogenicity for just those weeds without damaging other plants. They are selecting the most promising, aggressive pathogens and creating genetic variation panels to figure out what genes in the fungus promote virulence.

“Nature tends to create a balance,” he explains. “To thrive and reproduce, fungi don’t want to wipe out their host too quickly, so most pathogens aren’t that lethal. But some aggressive strains of a pathogen are capable of quickly killing their host. For herbicide purposes, those are the strains we are most interested in, but finding them is a numbers game.”

Once aggressive strains have been identified, genetic technology and tools allow Bluhm to pinpoint genes involved in the strength and aggressiveness of each pathogen. He also considers other traits relevant to a bioherbicide, like storage stability, spore production and more.

“We look at the variations we create and their parent strains to select specific characteristics to create a good bioherbicide, all within the pathogen’s natural genome,” he says. “As we find interesting characteristics, we are able to genetically map them in just weeks or months to explore further potential.”

Bluhm’s team has several fungi on parallel tracks of investigating variations and prioritizing the most promising options to explore further. He expects the process of reviewing fungal pathogens to find an ideal bioherbicide candidate to take a couple more years.

“Once the promise of a fungus is clear, we are accelerating efforts to understand it and learn if it can be developed into an herbicide,” he says. “The goal of our work is to lead toward an effective, practical, consistent, cost-effective commercial bioherbicide product for Palmer amaranth.”

He expects current stages of work to translate well to bioherbicides for other pigweed species and other weed families.

“This project has also been a great opportunity for student training,” Bluhm adds. “The process of screening pathogens and mapping their genes supports their education, helping them understand how new technology can be applied to real-world challenges.”

This project was funded by the soybean checkoff. To find research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.