EAST LANSING, Mich. — A leaf disease that has never been formally documented in the United States is showing up in Michigan corn, and Michigan State University scientists are asking farmers for help figuring out what it is.
The symptoms line up with a disease older studies describe as maize white spot, sometimes called Phaeosphaeria leaf spot. Once thought to be a local problem, it has now been spotted in about 16 counties, stretching from Berrien, St. Joseph and Van Buren in the southwest to Sanilac in the Thumb, and including Allegan, Barry, Calhoun, Hillsdale, Jackson, Ingham, Clinton, Livingston, Genesee, Shiawassee, Montcalm and Mecosta. Similar reports have come in from New York and Pennsylvania.
In some fields the spots have been both common and severe. Lesions begin as small pale green patches that bleach out and die, ringed by dark brown edges, and they can be round or stretched along the leaf.
The big unknown is the cause. Early DNA work has not been precise enough to pin down the species. Lab samples keep turning up a fungus that leaves tiny black fruiting bodies inside the lesions, but researchers caution that finding a fungus over and over in sick tissue does not prove it causes the disease. The MSU team is combining lab culturing, microscope work, DNA sequencing and infection tests to settle the question, which would let them build a reliable diagnostic test and survey how far the disease has spread.
Until then, there is no guidance on prevention or treatment, and the effect on yield and grain quality is still being studied. Past research elsewhere tied the disease to heavy rain combined with cool nights.
Farmers and the agronomists and consultants who advise them are asked to report suspect fields and sending in several fresh, symptomatic leaves along with the county, field location, hybrid, growth stage and any recent fungicide use. Leaves that cannot be shipped right away should be dried in a paper bag. Samples from counties not yet on the list are especially useful.
The disease adds to a slow harvest. As of Oct. 18, Michigan farmers had brought in 18% of the state’s corn, well behind the five-year average of 31%, according to federal crop progress data. About 51% of the crop was rated good or excellent.
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