by Carolyn Bernhardt
November 12, 2025
The state’s multibillion-dollar corn industry—and the farmers who depend on it— face a persistent, cold-hardy enemy: Corn tar spot. This plant disease has become one of Minnesota’s most troubling agricultural threats over the last six years since scientists first confirmed its presence in the state. The disease, which freckles corn leaves with black lesions, is caused by the fungus Phyllachora maydis. It was first detected in the United States in 2015, and in Minnesota in 2019, and has since spread to dozens of counties in Minnesota, slashing yields by as much as 70 bushels per acre in severely infected fields.
University of Minnesota scientists are working on a project to map the disease, understand its biology, and develop resistant corn hybrids. “We just don’t know much about this fungus so every step is a learning process,” says Dean Malvick, PhD, a professor in the Department of Plant Pathology at the College of Food, Agricultural and Natural Resource Sciences (CFANS).
“The fungus itself is really complicated,” says José Solórzano, a PhD candidate in Malvick’s lab. “The biology is a little complex — and not just the fungus, it’s the whole disease system that is complicated and tricky.”
With support from the Minnesota Invasive Terrestrial Plants and Pests Center (MITPPC), Malvick and Solórzano are leading the state’s only dedicated research on tar spot. For years now, their efforts have ranged from mapping the pathogen’s distribution to testing management tactics and identifying related Phyllachora species that might also infect corn or other plants.
Built to survive and evade
Initially, researchers believed tar spot required heavy rain, fog, and high humidity to thrive. But among their many findings, Malvick’s team has overturned that assumption. “It has become clear we don’t need those conditions. We don’t need that much moisture. It developed and infected quite nicely even though it was dry,” he says. The pathogen can also survive brutal Minnesota winters, enduring temperatures as low as –20 °C.
That resilience has fueled corn tar spot’s steady march across the Midwest and left scientists scrambling for answers. “This is an obligate fungus. It only grows on living corn tissue, so that’s a big part of the challenge, and even under the microscope we can’t distinguish [similar species] easily,” Malvick explains.
Fortunately, Solórzano and collaborators have developed groundbreaking tools to make this research more feasible. Using a method he jokingly calls “pimple-popping,” he extracts spores from tar spots to infect plants under controlled conditions and field environments. “We found that we can use the spores of the fungus and inoculate corn plants. We also learned how to store the fungus under freezing conditions to keep the fungus alive and use the spores for inoculations,” he says.
Malvick adds, “José is the first one to develop, use, and report a field inoculation protocol that works and is effective.”
Previous studies show tar spot can infect corn leaves and produce tar spot in as little as 10 to 16 days. Malvick has also helped create a DNA-based test that detects infections during the window before symptoms appear, giving farmers precious time to act.
“I think the surprising part of the research we are doing right now is that we know that this corn pathogen can infect other corn relatives, which was surprising to me. Those relatives are grasses, not corn, and are not present in the US,” Solórzano says. A broader host range could allow the disease to hide in unexpected places, complicating management strategies where those grasses are found. An experiment is underway to discover alternate host species including those that grow in Minnesota.
The disease’s behavior is inconsistent, too. “We can have severe disease in one area and 40 miles away much less severe,” Malvick says, underscoring how little is known about the factors that drive outbreaks. The first step towards stopping it is to understand how it survives and spreads.
A closer look
The team is also studying the role of a secondary spore type of Phyllachora, called conidia, which the fungus produces in the thousands. They suspect the spores play a key role in how the disease spreads, but the exact function is not known. “If the fungus produces them, they must have a function. So we are working to understand what their function is,” says Solórzano. The team also aims to refine management recommendations and support the development of corn hybrids resistant to tar spot.
Despite the challenges, both researchers remain optimistic. “People are starting to understand the disease. They know more about the disease cycle, they know more about the fungus, how to identify it—and that gives me hope,” says Solórzano.
Malvick agrees: “We have some strategies we know can be effective in both the short and long term. Developing protocols will lead to resistant corn hybrids that will help keep the disease in check, and that is already happening.”
As Minnesota farmers prepare for the next growing season, the work of Malvick, Solórzano, and their collaborators offers hope and practical tools for managing this resilient agricultural threat.
The Minnesota Invasive Terrestrial Plants and Pests Center research is supported by the Environment and Natural Resources Trust Fund, as recommended by the Legislative-Citizen Commission on Minnesota Resources.
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