Phaeosphaeria Leaf Spot Spread Across Michigan Cornfields
The fungal disease has reached 20 counties in Michigan, creating uncertainty for farmers as management options remain limited.
Updated on Oct. 9, 2026 in Botany
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Phaeosphaeria leaf spot has been confirmed in approximately 20 Michigan counties, marking a significant expansion of the fungal disease first identified in Florida in 1999. Researchers are currently isolating the pathogen to confirm the causal agent and assess potential damage.
Why it matters
The disease presents a new management challenge for regional agriculture because its symptoms overlap with common issues like paraquat herbicide injury and holcus spot, complicating early identification. With no currently labeled fungicides for treatment, researchers are working to develop resistance screening and management strategies.
Researchers report 95% confidence in identifying the fungal causal agent, which exhibits symptoms resembling paraquat herbicide injury and holcus spot. While tar spot caused yield losses of up to 50% in 2018 and 2021, the specific yield impact of Phaeosphaeria leaf spot is still being determined.
The details
Researchers are actively isolating the fungus from diseased corn tissue to study the pathogen's characteristics. Once isolated, the fungus is reintroduced to healthy plants in controlled greenhouse settings to verify symptom development and establish disease etiology. This process is critical for distinguishing the leaf spot from other physiological or chemical stressors currently present in Michigan fields.
Timeline
1999: Disease first reported in Florida seed production.
2016: Tar spot first discovered in Allegan County, Michigan.
2018: Severe tar spot epidemics occurred.
2021: Severe tar spot epidemics occurred.
October 2026: Confirmed spread of Phaeosphaeria leaf spot in Michigan.
The Tech Race
The emergence of this disease follows a pattern of fungal pathogen expansion in regional corn production similar to the documented impact of the 2018 and 2021 tar spot epidemics. Agricultural scientists are now racing to identify resistant corn hybrids and efficacy data for fungicides to mitigate potential damage.
Farmers currently have no labeled fungicides to combat the spread, meaning management is restricted to identifying affected fields. Researchers are now working to test potential fungicide efficacy and screen hybrid varieties to provide future management guidance.
The takeaway
The primary challenge remains distinguishing this disease from existing herbicide damage and foliar spots to prevent unnecessary treatment. Stakeholders should monitor for new guidance regarding emergency fungicide exemption labels as researchers finalize efficacy trials.
Further reading
For more information on plant pathogens, visit our Botany section.
Source note: This article includes information reported by Farm Progress.
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