NEW YORK (AP) – Federal prosecutors charged two Chinese researchers on June 3 with smuggling a crop-killing fungus into the U.S. last summer – charges that come amid heightened political tensions between the two countries and as the Trump administration moves to revoke visas from visiting Chinese students.

Yunqing Jian and Zunyong Liu are charged with conspiracy, smuggling, making false statements and visa fraud for allegedly bringing the fungus Fusarium graminearum into the U.S. Jian, 33, was booked in a Detroit federal court. Liu, 34, is thought to be in China.

According to the FBI, Liu had small baggies of the fungus stashed in his backpack when he flew to the U.S. last year and, after claiming ignorance about the plant material inside them, said he was planning to use it for research at a University of Michigan lab where Jian worked and where Liu previously worked.

Fusarium graminearum causes a disease called Fusarium head blight that can wipe out cereal crops such as wheat, barley and maize and rice – it inflicts $1 billion in losses annually on U.S. wheat and barley crops, according to the USDA.

It isn’t the only fungus to cause Fusarium head blight, but it’s the most common culprit in the U.S. The fungus infects plants early in the growing season, shriveling wheat grains and blanching crop heads a whitish-tan color. It also causes a toxin to accumulate in wheat kernels that can make them unsafe for people and livestock to eat.

Nicknamed “vomitoxin” because it’s most known for causing livestock to throw up, it can also cause diarrhea, abdominal pain, headache and fever in animals and people.

Wheat and other grain crops are screened for various toxins, including Fusarium graminearum, before they can be used to feed animals and humans. Farmers have to throw out any infected grains, which can cause devastating losses.

“It’s one of the many problems that farmers have to deal with that risks their livelihood,” said David Geiser, a Fusarium expert at Penn State.

Although Jian and Liu are accused of smuggling Fusarium graminearum into the country, the fungus is already prevalent in the U.S. – particularly in the east and Upper Midwest – and scientists have been studying it for decades.

Researchers often bring foreign plants, animals and even strains of fungi to the U.S. to study them, but they must file certain permits before moving anything across state or national borders. Studying the genes of a foreign fungus strain, for example, can help scientists learn how it tolerates heat, resists pesticides or mutates.

“We look at variations among individuals just like we do humans,” said Nicole Gauthier, a plant pathologist at the University of Kentucky who studies Fusarium.

That said, it’s unclear why the Chinese researchers might have wanted to bring that strain of Fusarium graminearum into the U.S. and why they didn’t fill out the proper paperwork to do so.