Phaeosphaeria leaf spot has been detected widely in Michigan cornfields, and agricultural specialists warn the emerging disease could reduce yields on a scale comparable to tar spot, which caused costly epidemics in 2018 and 2021. The disease, also known as white spot, has been confirmed in about 20 Michigan counties, according to Martin Chilvers, a Michigan State University field crops Extension specialist. It was first reported in Florida’s winter seed production in 1999 and is now appearing in the Midwest for the first time.
“This is giving me tar spot flashbacks to 10 years ago,” Chilvers said. He described the disease as widespread, noting he can find symptoms in nearly any Michigan cornfield he visits. Cases have also been identified in New York, with suspected detections in Pennsylvania and northern Indiana. Chilvers believes it may already be present in other Great Lakes states and potentially in Ontario and Quebec.
Symptoms and identification challenges
The symptoms of the fungal disease can closely mimic paraquat herbicide injury, which may explain why it escaped notice in previous seasons. Chilvers said it may also have been mistaken for holcus spot. He located a photo from a couple of years ago that appears to show the same symptoms, suggesting the disease could have been present longer than recognized.
Potential impact and concerns
Chilvers draws parallels between the current pattern and tar spot’s emergence. When tar spot was first seen in Allegan County, Michigan, in 2016, it appeared as isolated lesions that seemed manageable. By the following year, entire fields were heavily affected, and severe epidemics in 2018 and 2021 led to yield losses of up to 50 percent in the worst fields.
Under high disease pressure, white spot appears capable of significant damage. Reported consequences include:
- early senescence of grain corn
- stalk standability problems
- yield loss from reduced grain fill
Chilvers said many hybrids may be highly susceptible because breeding programs have not screened for resistance to this disease. “I’m pretty concerned,” he said.
Research and verification efforts
MSU researchers are running laboratory assays to confirm the causal fungus. The team is isolating the organism from infected leaves and reintroducing it to corn in greenhouse trials to reproduce the same symptoms. “I’m 95% sure, but we’ve got to do the science to prove this is the fungus that’s causing the white spots,” Chilvers said.
Cornell University and Purdue University have launched their own investigations, with MSU leading the effort. Several large agricultural companies have expressed interest in the results.
Outlook and management
The disease’s widespread presence suggests inoculum is now established across Michigan, raising concerns for coming seasons. “We don’t know what the impact’s going to be,” Chilvers said.
There are currently no fungicides labeled in the United States for Phaeosphaeria leaf spot. Companies are exploring emergency exemptions, similar to steps taken when tar spot first emerged. Chilvers noted that products used in Brazil have shown activity, but U.S. testing and approvals are still required.
Recommendations for growers
Identifying the causal agent is a critical first step for management, Chilvers said. Confirming the pathogen will enable development of a reliable diagnostic assay to support broader surveys and to map distribution and prevalence in Michigan and beyond.
The research team plans to begin screening corn hybrids for resistance and to evaluate fungicide efficacy as part of longer term management strategies. “We would love to know if growers encounter any issues, or if there is any suspected yield loss with this,” Chilvers said.
Phaeosphaeria leaf spot: Help document the disease
Growers can assist researchers by following these steps:
- Collect several representative symptomatic leaves, preferably with fresh or actively developing lesions.
- Record the county and general field location, hybrid, crop growth stage, and any recent fungicide applications or other relevant disease management information.
- If samples cannot be submitted immediately, dry them in a paper bag.
To report a potentially affected field or arrange a sample submission, contact Martin Chilvers. Samples may be sent to: Martin Chilvers, 578 Wilson Road, CIPS104, Department of Plant, Soil and Microbial Sciences, East Lansing, MI 48824.
Growers and agribusinesses tracking this emerging threat may also be interested in how new technologies are being deployed to protect crops from pathogens, as covered in Resurrect Bio AI platform targets crop disease defenses.