An enemy reunion

Nick Greatens, PhD student, suspects a deadly game of matchmaker could help eradicate invasive species.

August 25, 2021

by CAROLYN BERNHARDT | MITPPC communications specialist

The new crown rust on glossy buckthorn
Crown rust on glossy buckthorn | Photo by Nick Greatens

Could bringing two enemy species together help keep invasives in check? Nick Greatens has a hunch the approach might just work, so he’s using funding from the Minnesota Invasive Terrestrial Pests and Plants Center to investigate the concept in a specific context — using rust fungi to manage two major invasive species in Minnesota, glossy buckthorn and reed canarygrass. 

Greatens is co-advised by Pablo Olivera Firpo, PhD, a research assistant professor in the Department of Plant Pathology in the College of Food, Agriculture and Natural Resources Sciences, and Yue Jin, PhD, a research plant pathologist with the USDA and adjunct professor in the Department of Plant Pathology. Jin and Olivera-Firpo’s labs investigate the biology and population genetics of cereal rust fungi and rust resistance in cereal crops. 

Cereal rust fungi got their common name from the harm they cause to our cereal grains — wheat, oats, and barley. They endanger our food supply by infecting these crops, and they’re tough to manage because their spores travel through the air. “Oat crown rust regularly causes between 10 and even 50 percent crop yield loss in Minnesota and we have one of the biggest oat productions in the Midwest,” Greatens says. 

Greatens is pursuing a related strain of rust that’s relatively new to the U.S. He’s testing whether this new strain of crown rust can help land managers manage two invasive big hitters, glossy buckthorn and reed canarygrass. The research is in the preliminary stages, but it could lead to lasting solutions, especially if rolled out alongside other management practices already in place for the two invasive species. 

Powers for good

Rust fungi earned their name by infecting host plants with diseases that result in an orange, rust-looking appearance. Rust fungi only live so long as the plant they cling to lives, all the while impeding things like the plant’s vigor, ability to photosynthesize, and seed production. This new strain infects reed canarygrass and glossy buckthorn, two top threats to Minnesota’s wetlands, with ease. However, Greatens says, “With biocontrol agents, they are most effective when they are used with other management practices.” So, if the scientists can prove that the specific crown rust they are studying is an effective tool for managing reed canarygrass, the approach would still have to be combined with other practices, like mowing, burning, and pulling grasses, to further advance the overall management of these pesky plant species. 

When it comes to managing glossy buckthorn, though, Greatens says reed canarygrass would have to act as the conduit for the fungus. “Managers would have to hay the afflicted reed canarygrass and spread it over the buckthorn. If it were ever to be implemented on a large scale, it would be a pretty involved process of cultivating the fungus, haying the grass, spreading it on the field or grinding it up and spraying it on the buckthorn.” No one has ever done any of this as a land management practice before so Greatens says, “It’s a little bit of a wacky idea.”

Scientists are using all kinds of fungi for controlling other invasive pathogens, though. Relatively few, however, are employing fungi as augmentative biocontrol agents of weeds, meaning manipulating a pathogen that’s already present and appears to curb invasive species growth. The approach is common in managing pathogens, but using it on weeds is fairly new. These kinds of biocontrol agents are sometimes called bioherbicides. Since this type of crown rust is likely non-native, it’s a delicate process.

An added bonus of the project is that an overall better understanding of rust fungi could lead to improved food security, too. “Maybe by studying this fungus we can understand a little more about these major cereal pathogens in oats and barley, too,” Greatens says. “We could learn more about their genetics and get a better idea of how these things are working and, eventually, ways to manage them.”

A complicated past, present, and future

“This crown rust is probably an introduced species,” says Greatens. “It’s hard to know for sure, it could have just had a low population previously and then recently exploded. But most likely it was introduced here from Europe, where it has been described and studied for up to 120 years.” It’s possible, he says, that the new strain came to the United States about 10 years ago. The rust is most common in the eastern part of the United States and the Midwest. In Minnesota, the rust is most common in the eastern part of the state. It is not currently found on the western Minnesota prairie, or around Duluth and the North Shore where glossy buckthorn is uncommon. But in the Twin Cities, it’s ubiquitous.

Fortunately, this new strain of rust does not appear to affect Rhamnus alnifolia (native buckthorn in Minnesota). Although, it can harm some native buckthorns from other regions of the country, so deploying it, Greatens says, should be managed carefully.

“Native buckthorns with a more southern distribution and in California could be affected by this rust fungus very clearly, based on controlled studies,” Greatens says. “So, if this is an exotic pathogen — and it probably is — it is likely to affect some native species including these native buckthorn species in the eastern U.S.” 

Based on tests Greatens and team have done in the greenhouse, the rust fungus is likely to affect a few other native grass and buckthorn species across the country, which Greatens says would likely be enough to disqualify it as a classical biocontrol agent. Classical biocontrol agents are specifically brought into the country to help manage invasive species. So, this crown rust would not be a control measure scientists could ship in from Europe because it poses too much risk on certain native species. “But since it’s already here and apparently somewhat limited in its ability to spread far beyond the range of glossy buckthorn, it could still have potential as an augmentative biocontrol. Using crown rust as an augmentative biocontrol is unlikely to affect its impact on another buckthorn species 600 miles away.” Greatens is uncertain whether the rust has traveled west of the Rocky Mountains yet. 

So far, the team has seen that the rust effectively keeps reed canarygrass and buckthorn in check in a greenhouse setting. In the field, the rust clearly reduces glossy buckthorn’s ability to produce seeds in early summer and can cause both hosts to lose leaves. However, Greatens says the researchers are still working to measure its effect in the field. “We have been testing a lot of native species in the greenhouse to see if they are susceptible to the fungus,” says Greatens. The team also needs to identify a seamless method for land managers to inoculate plants in the field with this fungus. 

The project is funded by MITPPC for three years and is halfway complete. The research should wrap up toward the end of 2022, but establishing a plant or fungal species as a biocontrol is a lengthy process, sometimes taking many years to get approved by the USDA. Still, this research to help measure the rust’s efficacy in the field will be essential to even get that process started.

Opposites attract

The larger question is whether this game of what Greatens calls “enemy reunion” is a new tool researchers can add to their ever-growing tool-kit for beating back invasives. “So glossy buckthorn got here from Europe and has been proliferating for 200 years and causing problems,” Greatens explains. “One of the reasons it did that is because there are fewer enemies in its introduced range here — fewer insects and pathogens — because it has been separated from its context where they co-exist.” 

So, Greatens says this new rust coming over is an example of one of glossy buckthorn’s natural foes coevolving in Europe and being reunited with its host in its introduced range. And it turns out, this landscape soap opera is common in all kinds of systems and contexts. “It’s not a very well-known phenomenon,” says Greatens. “Part of the novelty of this research is looking at what can reasonably be considered an invasive species—the fungus—being spread to North America from Europe and strangely affecting other invasive species that are all also present in Europe.”

The unique story of this exotic organism coming to North America and having a positive effect, at least locally, is likely to shape future research on new methods for managing invasive species.