Weed & Pest
Microbial control for cheatgrass researchedWritten by Saige Albert
Soil Microbiologist Ann Kennedy of the USDA Agriculture Research Service’s Land Management and Water Conservation Research Unit in Pullman, Wash. has been working on bio-control methods for cheatgrass.
“I work on some interesting bacteria,” Kennedy said in her presentation at the Wyoming Weed and Pest Council Fall Conference and Business meeting in Casper on Nov. 7. “These bacteria have the ability to inhibit grass weeds.”
Kennedy’s work began when she was a post-doctorate student looking at trends in winter wheat.
“We found that early in the spring, we were seeing yellowing of winter wheat in depressions,” she explained. “Plants don’t all grow at the same rate, and we were curious what caused the yellowing.”
They delved into the roots and soil to find a bacterium that was colonizing the roots of the plant.
“We found that 90 percent of the roots of stunted plants were colonized with the bacteria,” she said. “We thought, if we can find those bacteria that selectively inhibit the roots, we can use them, and we did find this organism by selecting for them.”
“The soil is alive,” commented Kennedy. “We have a crew of bacteria in the soil working for us everyday, and the majority of soil microorganisms are beneficial.”
She added that in selecting a bacterium for use as a bio-control mechanism, it was important that there was no inhibition of economically valuable crops or native plants. In order to accomplish that goal, the scientists utilize a labor-intensive selection system.
“We isolate the soil and bacteria in March, when they are most active,” Kennedy explained. “They don’t grow everywhere or all the time, and they are very selective.”
Using a bioassay, they grow the bacteria, and Kennedy says, “If it doesn’t inhibit a weed, we throw the bacteria away. We keep the ones that inhibit the weeds and bioassay against beneficial plants.”
The screening process puts the bacteria up against wheat, native plants and dicots, among others.
“If it hurts an economically valuable or native plant, we get rid of it,” Kennedy explained. “The ones that don’t affect them, we keep.”
In locating this particular bacteria, she noted that they started with over 10,000 bacteria isolates, ending with only about one percent that inhibit weeds but don’t affect the beneficial plants.
“It is a very arduous process,” she said. “We make sure it doesn’t have any antimicrobial or antifungal activity, and we make sure the genes are on the chromosomes so they don’t transfer.”
Screening also ensures that the product is ecologically safe, won’t result in any residue and are only active at the intended times.
“We have a good match for bio-control now,” she said. “We have a bacterium that inhibits root growth at cool temperatures.”
The bacterium, a variety of Pseudomonas, thrives in cold soil temperatures, making them ideal to target cheatgrass species.
“Cheatgrass is such a problem because it has low temperature root growth,” explained Kennedy. “There is a wonderful root mass that is being accumulated, and it growers further in the winter and starts earlier in the spring – that is why it is such an invasive plant.”
In addition to cheatgrass, the same bacteria also targets jointed goat grass and medusa head, which are also important invasive species in the West.
The competitive advantage of these species is gained from their ability to produce vast root systems in cool-weather temperatures, which is when the bacterium is active.
The bacterium essentially prevents root growth and does not allow seeds in the soil to send up shoots by entering the root system of the plant and producing a complex toxin. Because of their location inside the root of the plant, no toxins are released into the soil, making the product more environmentally safe.
“The thing is, we need the patience to wait a couple of years for the bacteria to really work,” Kennedy explained of the organism. “We have to get this organism into the soil and growing in the soil before it does its thing.”
In trial work, the product took about four years before cheatgrass was nearly eliminated from test plots.
“We have four different sites, and after we applied bacteria, we found that in the first year, we only saw a 10 to 20 percent inhibition,” she explained. “As we go through, the bacteria started reducing the competitiveness of the cheatgrass, and the natives began to reappear.”
Through her fieldwork, Kennedy noted that by four or five years after application, cheatgrass disappears.
“In some places, we can get about 100 percent control in three years,” she added.
Kennedy also mentioned that the bacterium can be used in combination with herbicides for additional control efforts.
Currently, Kennedy noted that the bacteria will be a low-cost product for effective weed control, but the bacteria must survive to be effective. They must also enter the soil.
“We put the organism on in the fall or winter when temperatures are lower and when a rain or snow is going to come,” Kennedy explained. “A little rain is necessary to get the bacteria off the surface of the soil and in deeper.”
She also added that the bacteria must establish in the soil.
Additionally, for the bio-control method to work, native species must be able to come back and compete.
“We have to manage this more than just spraying herbicide,” she commented.
Kennedy continued that currently, the U.S. Fish and Wildlife Service is working to register the product and get it approved for use through the Environmental Protection Agency, but the potential implications of a bio-control mechanism are important.
“This is a really, really interesting tool to put in the toolbox,” Kennedy mentioned. “If we can find organisms in the soils to do the job and inhibit specific plant species, we can have some good bio-control matches.”
“We need to improve plant diversity and get cheatgrass out,” she continued. “We aren’t talking about the silver bullet – we are talking about silver buckshot. We need a shotgun approach, with all these things working together to get to our target – which is getting rid of cheatgrass.”