Michigan State University researchers are investigating an emerging leaf spot disease in corn that has been reported in approximately 14 Michigan counties during the 2026 growing season, with additional reports from New York and Pennsylvania.
The disease produces symptoms resembling those historically associated with Phaeosphaeria leaf spot (PLS) or maize white spot (MWS). Initially believed to be a localized problem, the disease has been observed with notable incidence and severity in affected fields.
Researchers have not yet identified the fungal species responsible for the disease. Historical literature has associated PLS and MWS with Phaeosphaeria maydis, but the identity and taxonomy of organisms referred to by that name remain unresolved.
Recent examinations of Michigan samples have consistently identified a phomoid fungus in symptomatic leaf tissue. Its fruiting structures, known as pycnidia, appear as small black spots within the lesions and can be seen under magnification. However, preliminary DNA sequencing has not provided sufficient information to confidently identify the species.
MSU researchers are seeking additional samples from affected cornfields to help identify the organism and determine the disease's distribution across Michigan.
Their investigation combines fungal isolation, morphological examination, DNA sequencing and pathogenicity testing to identify the organism responsible for the observed leaf spots. Researchers caution that symptoms alone are insufficient for identification and that repeatedly detecting a fungus in symptomatic tissue does not establish it as the causal agent.
Identifying the organism will help researchers better understand the disease, develop management strategies and establish a reliable diagnostic test. Such a test would support broader surveys to determine the disease's distribution and prevalence in Michigan and other states.
Researchers are asking growers and agricultural professionals to collect several representative symptomatic leaves, preferably those with fresh or actively developing lesions. Samples should be accompanied by information about the county and general field location, corn hybrid, crop growth stage and recent fungicide applications or other relevant disease-management practices.
If samples cannot be submitted immediately, they should be dried in a paper bag.
Researchers also are seeking information about disease severity and any potential effects on yield or grain quality in affected fields.
Source: Michigan State University, "Emerging leaf spot disease of corn under investigation in Michigan"