Phaeosphaeria leaf spot, a fungal disease previously unreported in the Midwest, has been confirmed in about 20 Michigan counties, with suspected cases in Pennsylvania and northern Indiana and likely spread into other Great Lakes states plus Ontario and Quebec.

Michigan State University field crops Extension specialist Martin Chilvers told Agribusiness Wire the pattern mirrors early tar spot, which after a 2016 discovery in Allegan County, Mich., triggered epidemics in 2018 and 2021 that cost growers yield losses of up to 50% in severely affected fields. Combined losses from those two seasons ran into the tens of millions of dollars across affected operations.

"This is giving me tar spot flashbacks to 10 years ago," Chilvers said. "I am concerned it's going to be more like early days of tar spot with significant impacts. Basically, any cornfield I walk into here in Michigan, I'm able to find it. So, it's widespread."

How widespread is the outbreak?

MSU has confirmed white spot in roughly 20 counties. Cornell University has identified it in New York. Suspect samples have surfaced in Pennsylvania and northern Indiana.

Chilvers believes inoculum has reached additional Great Lakes states and the Canadian provinces of Ontario and Quebec.

The pathogen first surfaced in Florida winter seed production in 1999. Its Midwest debut this season marks a geographic jump that agronomists are still mapping.

What damage can growers expect?

Chilvers cited four probable agronomic consequences in fields with high disease pressure:

• Rapid dry down of silage
• Early senescence of grain corn
• Stalk standability issues
• Yield loss from reduced grain fill

The standout risk is stalk integrity. Weak stalks complicate harvest timing and can drag down test weight, tightening margins already pinched by input costs.

"We probably have a lot of hybrids that may be very susceptible to this disease," Chilvers said, because seed companies never screened commercial lines for white spot resistance. The unknown genetic vulnerability across the seed trade is the biggest open variable.

Why did it go undetected?

Symptoms mimic paraquat herbicide injury, which Chilvers said may have masked the disease in earlier seasons. Holcus spot, a separate fungal issue, presents similarly, raising the chance that prior white spot cases were misdiagnosed.

Chilvers located a photograph from two years ago showing lesions consistent with the current outbreak, suggesting the pathogen may have circulated in Michigan longer than this season.

What is the research status?

MSU researchers are isolating the fungus from diseased tissue and reintroducing it to greenhouse plants to confirm causality. "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 and Purdue have begun parallel investigations. Several seed and crop protection companies have contacted MSU about the findings, Chilvers added, signaling commercial interest in rapid diagnostic and resistance work.

A reliable diagnostic assay is the next deliverable. It will let researchers run broader surveys across the Midwest and refine distribution maps that currently rest on extension reports and field scouting.

What management options exist?

No fungicides carry federal labels for Phaeosphaeria leaf spot. Companies are exploring emergency exemption labels, the same regulatory pathway used during tar spot's early years. Fungicides used in Brazil have shown efficacy, but U.S. trials and approvals remain pending.

Until labels arrive, growers lack a chemical playbook. That gap heightens the importance of hybrid selection and residue management heading into 2026.

How can growers help?

MSU asks producers to send several symptomatic leaves with active lesions, county and field location, hybrid, growth stage and recent fungicide records. If samples cannot ship immediately, dry them in a paper bag.

Ship to Martin Chilvers, 578 Wilson Road, CIPS104, Department of Plant, Soil and Microbial Sciences, East Lansing, MI 48824.

"We would love to know if growers encounter any issues, or if there is any suspected yield loss with this," Chilvers said. "We're just trying to learn at this stage. Like, what does this mean?"

Inoculum is now established across Michigan, and the 2026 growing season will test whether white spot follows tar spot's escalation curve from isolated spots to whole-field losses.