Growing up in Peru, Diego Gutierrez, a graduate student at Colorado State University, formed a deep connection to agriculture. His grandmother owned a potato farm in the Peruvian highlands, and his father worked at a facility that processed the country’s sought-after coffee beans. Those close ties meant that Gutierrez also learned early on about the disastrous impact an uncontrolled plant disease can have on a crop and a community.
When he was a teenager, a fungal infection known as yellow rust spread through local coffee fields. The crop was so hard hit that Gutierrez’s father struggled to continue to find work at the processing plant. “So,” Gutierrez said, “I’ve always been interested in plant pathology and trying to find and diagnose disease before it becomes a problem that can affect farmers’ livelihoods.”
This fall, that’s exactly what Gutierrez did. He discovered and identified a new bacterial disease affecting wheat plants in at least five counties in eastern Colorado. Gutierrez made the discovery while working toward his doctoral degree in a lab run by Robyn Roberts, an assistant professor in CSU’s Department of Agricultural Biology. “It feels like a big moment,” Gutierrez said, “finding something in the early stages.”
What Gutierrez and Roberts found is a disease known as leaf blight. The leaf-attacking disease is caused by bacteria called Pantoea, which had previously only been reported to affect wheat in China and parts of eastern Europe. (Pantoea has been found to attack rice and soybeans in the United States, among other agricultural crops.)
Gutierrez and Roberts followed a thorough review process, including genetic mapping and consulting with the Colorado Department of Agriculture (CDA), to confirm the discovery. Roberts then worked with CSU colleagues and CDA to report the finding to the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service, the federal regulatory agency that monitors invasive pests and diseases. Roberts also notified the Colorado Wheat Administrative Committee, a statewide association of wheat farmers.
At the moment, Roberts doesn’t anticipate leaf blight will cause too much trouble. Best case scenario, she said, is that it’s been around for a while and no one found it because it hadn’t been problematic. Chances are that the environment shifted in a way that triggered the microbe’s ability to infect Colorado-grown wheat. “Some microbes are just kind of ubiquitous and opportunistic,” Roberts said. “When conditions are right, they become pathogens.”
A light-bulb moment
This kind of discovery is precisely the work of the Roberts lab, which attempts to better understand plant immunity and pathogens to help prevent major epidemics. Roberts focuses specifically on wheat and other field crops that are important in Colorado and beyond. “My research is strongly driven by the needs of the growers in Colorado,” Roberts said. “One of our big goals is to detect pathogens before they become a significant issue.”
The Pantoea discovery happened almost by accident. Gutierrez was looking for another plant disease called bacterial leaf streak. Leaf streak has been around since the early 1900s but has reemerged in the Midwest in the past 10 or 15 years, creating problems for farmers in Nebraska, Kansas and elsewhere. The disease can cause significant yield loss, and, depending on the variant, there are few options for controlling it. Gutierrez and Roberts figured leaf streak had likely already made it to Colorado, but that no one had picked up on it.
Gutierrez designed his thesis around studying the potential for bacterial leaf streak to impact the Centennial State. On a pre-planned field research day to the Eastern plains last year, Gutierrez thought he found it. He collected samples from multiple wheat fields that showed symptoms of leaf streak. He tested the samples, but everything came back negative. Not thinking too much more about it, Gutierrez stored the samples in a freezer and returned to his thesis.
Months later, at a regular lab meeting where researchers discuss their work, a colleague mentioned that a new bacterial pathogen called Pantoea had been found infecting U.S.-grown rice and soybeans. Gutierrez thought the symptoms sounded a lot like what he’d observed in eastern Colorado. He texted Roberts his light-bulb thought during the meeting; they connected afterward, and Roberts said she’d been thinking the same thing.
With the help of two undergraduate students, Libby Swanson and Emma Barrett, Gutierrez went back to those tissue samples in the freezer, isolated the bacteria and had it genetically sequenced. Sure enough, one of the samples he had collected was a 100% genetic match for the strain of Pantoea that had been reported in China. What’s more, another sample matched a different strain of Pantoea that had never been reported to infect wheat.
Plant health is public health
Amy Charkowski, research associate dean in the College of Agricultural Sciences, shares Roberts’ feeling that the early signs suggest Pantoea will likely not cause significant issues in Colorado. Nevertheless, she said, the discovery itself is critical. “Plant health is basically public health,” Charkowski said. “And if you don’t have a problem identified, you can’t really manage it.”
Unlike with, say, an invasive beetle, farmers can’t just spray a field with pesticides to control a bacterial pathogen. There are some known controls for Pantoea that work with onions, but no research has been done to determine what might work with wheat.
Climate change and global trade can further complicate the work of containing a disease, Charkowski said. “Plant resistance genes are often controlled by temperature — if it gets too warm, they don’t work very well,” she said. “And then all of these microbes are affected by temperature, too; as the climate changes we’re going to see microbes that couldn’t previously survive do well.”
Roberts hopes to find additional funding to keep an eye on Pantoea. One immediate question is whether it can be transmitted on seeds. “If it’s traveling on seed, it’s a big concern,” she said. “It could be broadly distributed very quickly.”
What’s more, a changing climate could present new challenges. “There’s always the potential that if conditions change, or the variety changes enough,” Roberts said, “it could become an issue.”
For now, though, Gutierrez is focusing on his bacterial leaf streak thesis. But he’s glad to have stumbled upon the Pantoea discovery; it reminded him of why he wanted to do this work in the first place. “It’s definitely rewarding for me to see the research that I’ve done having an impact,” Gutierrez said. “Hopefully this can help farmers.”
