Tumbling along

 

What smashes into cars on the highway, spreads wildfire and causes painful weltering scratches? It's Russian thistle Salsola spp., more commonly known as tumbleweed, a hard-to-control invasive species that grows in disturbed soil and spreads quickly when the thorny plants break off from the ground and roll along dispersing seeds and piling up along fences and buildings. But now two fungi, one that eats dead plant cells and other that looks like rusty metal, could offer a way to fight back against the troublesome weed.

For 13 years Bill Bruckart and Dana Berner at the Foreign Disease-Weed Science Research Unit of the U.S. Department of Agriculture Agricultural Research Services center in Maryland have been studying the rust fungus Uromyces salsolae and a necrotrophic fungus Colletotrichum gloeosporioides f. sp. salsolae or CGS, which they believe show promise to control Russian thistle. But there are still barriers ahead that could prevent their research from resulting in a breakthrough treatment, including some people's love of the weed as an icon of the American West.

Russian thistle, which came to North America from Europe and Asia over 100 years ago, dries out soil, hosts crop diseases and threatens fences and buildings when it piles up against them ready to burn. Today there are at least six species occupying as much as 100 million acres of disturbed land, especially in arid Western states. Previous efforts using insect control to slow the invasive's spread have had little effect.

"Many of the introduced pests in the US have come in without their full complement of natural enemies," USDA's Bruckart explains. So researchers look for solutions in the plants' native habitat, eastern Europe and Russia. After finding pathogens that preyed on tumbleweed, the researchers studied them in a sealed greenhouse in the US. They spent decades testing whether the pathogens could harm more than 60 species of other plants to see if there was risk of the fungi infecting non-target species. They also conducted field studies in the country of origin, comparing plots of healthy tumbleweed with infected plots. In one such study in Greece, CGS killed nearly 100 percent of tumbleweed in a plot over two years.

Now that they have found likely fungal candidates for invasive control, the researchers are seeking regulatory approval from USDA's Animal and Plant Health Inspection Service. But despite the scientific evidence in favor of the release, regulators are hesitant to authorize application of the pathogens unless the public convinces them it's necessary. "The regulators are super risk averse," says Berner. "If there is no push from the end user, they find no need to be proactive."

Pushing back against their efforts is the image of tumbleweed as Western icon, complete with a whistling soundtrack and gunmen reaching for their six shooters at high noon. If you're feeling nostalgic for the wild West, you can buy your very own Huge Tumbleweed online for $39 plus $50 shipping and handling at buytumbleweednow.com. One online store selling "decorative dried wheat, plants, and grasses" promises, "For every dollar donated, Curious Country Creations will spread seeds so that tumbleweeds can continue with the history of the west [sic]."

The researchers hope public opposition to the invasive overcomes the nostalgia, and that regulators understand the need for the tumbleweed-terminating fungus. "(We) get frustrated with regulators," says Berner. The petition for the rust fungus has been awaiting approval for more than a year. He hopes those who have experiences with tumbleweed's destructive nature will "provide testimonials on how badly they do need" a bio-control.

"Tumbleweed is a big issue in the West," adds Bruckart.

Even if they get their permit to release fungus on the invasives, the researchers can't predict how effective the treatments will be as a control. "If it's too cold or too hot it doesn't work," Bruckart explains. Pathogens have been applied in Australia and other countries to successfully put a damper on invasive species, though, and Berner and Bruckart hope their work will achieve similar results in the Western U.S.

"The CGS would be a breakthrough I'm convinced," says Berner. "[It's] a darn good pathogen to control tumbleweed."

 

Tumbleweed image courtesy Flickr user Denise Rowlands.

Map of counties affected by Salsola tragus, one common species of Russian thistle, from Early Detection & Distribution Mapping System at the University of Georgia - Center for Invasive Species and Ecosystem Health. Available online at http://www.eddmaps.org/.

Emilene Ostlind is an editorial fellow at HCN.

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