Research HighlightsCreating a Palmer Amaranth Bioherbicide from Fungal Pathogens
In this article, you’ll find details on:
- University of Arkansas research has identified promising pathogens that could be developed into a bioherbicide to control Palmer amaranth.
- Combining molecular genetics and field trials, current research aims to determine how to most effectively deliver and apply a new bioherbicide.
By Laura Temple
Palmer amaranth has become weed enemy No. 1 in many soybean fields. Though it has proven the ability to develop resistance to herbicides quickly, it does have natural enemies — fungal diseases.
Burt Bluhm, professor of plant pathology at the University of Arkansas, is leading a team to turn at least a few of those natural enemies into a bioherbicide, thanks to soy checkoff funding from the Arkansas Soybean Promotion Board. He describes bioherbicides as using a living organism or its derivatives to control weeds.
After a few years of screening Palmer pigweed for plant-specific pathogens in the first phase of this effort, Bluhm has identified a few very promising options.
“We have two groups of pathogens that we are pursuing, which we call AF 22 and 24 and PWA 43 and 48,” he says. “Both have proven to be very host-specific to Palmer amaranth and lethal to young plants.”
He adds that the screening process for additional pathogens continues, though it gets less attention.
“It’s like a treasure hunt,” Bluhm notes. “You never know when the next great thing will pop up.”
Targeting Palmer Amaranth
When screening for potential bioherbicides, Bluhm looks for pathogens that cause a fungus that kills host roots or stems, preventing the weed from producing seed.
“Many pathogens make the host sick, but they don’t completely wipe it out so the fungus can survive to reproduce,” he says. “We are interested in the most aggressive, virulent pathogens that do kill a specific host.”
Bluhm explains that pathogens and plants often evolve together. Over time, plants develop natural defenses to a pathogen, and the pathogen in turn develops in ways that overcome those defenses.
“In this evolutionary ‘arms race,’ some fungal pathogens have evolved with a very narrow host range,” he says. “They become completely uninterested in other plant hosts. So far, our research shows that the strains we are exploring have done this with Palmer amaranth. However, we have to be sure anything we develop won’t bother crops.”
Based on his team’s genetic mapping of the two groups of promising fungal pathogens, Bluhm believes they are species that haven’t been previously described. However, genetic information reinforces their potential.
He explains that one group appears to be related to the first bioherbicide discovered and developed through the University of Arkansas more than 30 years ago. The other group appears to be related to a group of pathogens known to be very host-specific, though each identified species targets very different host plants.
Exploring Application Options
Trials with these promising pathogens are moving from the greenhouse into the field in 2024. Environmental factors in the field may complicate and influence results compared to the controlled environment of the greenhouse. Bluhm and his team will also compare application methods and timings.
“The PWA strains could be applied as pellets or a spray,” he says. “How they work in the field could be a reflection of how they are applied.”
Because the pathogens will be applied as living organisms, application logistics need to ensure they remain viable. This research will show what methods and conditions allow the pathogens to find and attack Palmer amaranth in the field.
“In the greenhouse, Palmer amaranth infected with the AF pathogens look like they’ve been sprayed with a flame thrower,” Bluhm reports. “These pathogens appear similar to others that produce a host-specific toxin. We are looking at how to apply the pathogens, but we are also determining if that toxin can be applied directly as a bioherbicide.”
He notes that his team is also investigating options for a cocktail that would combine the most effective Palmer-specific pathogens to overwhelm the weed.
His goal is to develop a first-generation bioherbicide from these pathogens that could be effective as they have been found through his team’s screening process. However, he believes that gene editing technology could be used to further weaponize the selected pathogens and increase their effectiveness for second- and third-generation bioherbicides.
“Fungi often carry genetic regulators that balance their aggressiveness with allowing the host to support their growth at least until they produce spores,” he explains. “With gene editing, we could turn off such genes to make the pathogens more virulent. However, such efforts likely come with tradeoffs, like the ability to overwinter. We will need to fully understand the impact of any changes before releasing them.”
He believes the first bioherbicide will require a few more years of research, followed by time to move through the regulatory process. Then he expects the University of Arkansas to license the technology to a commercial partner for production and distribution.
“This approach could be used on other problem weeds, and our library contains options that could open doors for new ideas in other areas,” Bluhm says. “Plus, this type of work is great training for students. It includes everything from grower-facing work and field trials to molecular genetics.”
Additional Resources:
Searching for a Palmer Amaranth Bioherbicide – SRIN article
Palmer amaranth research – SRIN page
Meet the Principal Investigator on this project: Burt Bhulm
Published: Jan 20, 2025
The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.
