URBANA, Ill. — Tar spot, caused by the fungal pathogen Phyllachora maydis, is an emerging disease in Northern Illinois. Prior to its identification in the United States, tar spot had only been reported in Central America, Mexico, and the West Indies. Since it was first identified in Illinois in 2015 on production dent corn, tar spot has been confirmed in 14 counties in north-central Illinois. Cool, humid conditions during the later stages of crop development are favorable for disease development on wet leaves with corn being the only known host for this pathogen.
Tar spot symptoms are quite distinct, with infection resulting in small, raised, black spots (ranging from circular to oval) on infected host tissue. The dark spots that form are called ascomata, which contain fungal spores that spread through wind storms and rain splash. The spots are surrounded by a tan or brown halo and may appear on both sides of the leaves, leaf sheaths, and husk; these spots can be found on both healthy and dying tissue. If you run your finger across the leaf of a corn plant infected with tar spot, you will feel tiny bumps where the dark lesions are. Other pathogens may be confused with tar spot, including corn rust (in the overwintering stage) and other saprophytic fungi which feed on dead corn tissue.
At this time, it is unknown if the pathogen is able to overwinter in Illinois, or if the typical growing season for this region will be conducive for infection and disease development. On susceptible genotypes, this pathogen may cause plants to produce ears with reduced weight and loose kernels and there are currently no fungicides labeled for tar spot in the United States. Despite the potential grain quality impacts, tar spot of corn is currently considered to be a minor problem with no evidence that it results in significant yield loss to producers in Illinois. Regardless, more information is needed to determine potential impacts, distribution across the state, and treatment strategies.
The University of Illinois Plant Clinic (https://web.extension.illinois.edu/plantclinic/) is trying to track and study this new disease, and tar spot infected samples of corn are needed for a new research project. If you think you have found corn tar spot, please collect several leaves showing symptoms and send them to the plant clinic at no cost, see how at this link (https://uofi.app.box.com/v/corntarspot). For questions regarding tar spot, or assistance with identification or sample submission, contact Phillip Alberti, Crop Science Educator with University of Illinois Extension in the College of Agricultural, Consumer, and Environmental Sciences at email@example.com, 815-235-4125 or on Twitter (@NorthernILCrops).
— University of Illinois Extension
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