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News Release

May 11, 2006

Weed Using 'Chemical Warfare' to Poison Fungi, Harm Trees, Study Finds

An invasive weed that grows abundantly in Canada and the United States is engaging in underground chemical warfare, jeopardizing native trees by poisoning their best allies, a University of Guelph professor has discovered.

The research by U of G ecologist John Klironomos, along with Kristina Stinson of Harvard University and a team of other researchers from Guelph, the United States and Germany, was published in the May issue of the Public Library of Science.

The scientists found that the weed garlic mustard (Alliaria petiolata) releases chemicals under the soil that are detrimental to the fungi that native trees — including the beloved Canadian maple — depend on for growth and survival.

Once garlic mustard produces these compounds and has invaded an area, even removing the weed doesn't help much. "The compound is still in the soil and it's hard to plant any native plants in the area and have them establish properly," Klironomos said.

It’s the first study to show that invasive plants are hurting indigenous species by thwarting the ecological relationship between roots and certain fungi.

“This noxious weed is disrupting an intimate symbiosis between native species and fungi that has been going on for millions of years,” he said.

Garlic mustard targets and poisons arbuscular mycorrhizal fungi, the main fungal allies of native trees like maple, ash and other hardwoods. The fungi have long microscopic threads that create a subterranean network, allowing for the exchange of nutrients with indigenous trees. The fungi rely on the trees for energy, and the trees rely on the fungi for food. “Forest plants are very dependent on these mutually beneficial relationships,” he said.

Klironomos noticed that some native tree seedlings had declined in forests where garlic mustard was present and suspected that the alien weed was the culprit. The researchers tested the theory by collecting soil from five Ontario forests containing both native hardwood trees and garlic mustard.

They planted seedlings in both infested soil and garlic mustard-free soil and studied the young trees’ ability to form relationships with fungi. The seedlings planted in infested soil grew at about one-tenth the rate of the other trees and had fewer fungal-root connections.

The researchers conducted the test numerous times to mimic different conditions in the wild: established forests, open fields, roadways, ditches, etc. Each time, the finding was the same: growth was stunted due to the diminished microbial activity caused by the presence of garlic mustard. “This is affecting current and future generations of trees and changing the habitat,” Klironomos said.

Mature forest systems are normally resistant to invasive weeds, largely because of the strength of fungi-tree relationships, he said. Fungi are so efficient at extracting nutrients from the soil that weeds like garlic mustard, which don’t form symbiotic partnerships with fungi, typically don’t stand a chance.

But garlic mustard’s guerrilla tactics have allowed it to get a foothold in some well-established forests. “There are entire carpets of this weed in some places,” Klironomos said. “It has found a way to survive by knocking out the competition.”

The researchers next plan to determine what chemicals in garlic mustard are killing the fungi, how these chemicals interact with other soil microbes, and how plants and fungi co-exist with the noxious species in its native European habitat.

Klironomos, a U of G faculty member since 1996, holds a prestigious Canada Research Chair in soil biology and was one of six Canadian scientists to receive a 2006 Steacie fellowship from the Natural Sciences and Engineering Research Council (NSERC), which allows him to focus full-time on research for two years. He has also received an Ontario Premier’s Research Excellence Award and other NSERC discovery and strategic grants, which helped support this research.