Tar spot was first identified in 2015 in seven counties in northwest Indiana and in 10 counties in north-central Illinois. In 2018, tar spot has been confirmed in Illinois, Wisconsin, Michigan and Indiana, according to University of Illinois extension.
The disease is caused by the fungus Phyllachora maydis and Monographella maydis. The most favorable conditions for infection are cool temperatures with high humidity along with sustained long-periods of leaf wetness. Here’s a link to the Purdue Extension bulleting on Tar Spot.
The yield impact of tar spot is being researched as well as potential future controls. Here is a link to a document compiled by the University of Illinois with known hybrid resistance.
Researchers are making a call for all farmers to send samples of suspected tar spot, and they should include valuable information such as hybrid, if a fungicide was applied and county of origin. Contact your extension or University pathology lab for more details.
Identifying the disease can be tricky as it mimics others, such as corn rust and tropical rust. The first symptoms of infection are brownish lesions on leaves, which then become black circular lesions (that have a similar appearance to a spot of tar). According to this Pioneer bulletin, up to 80% of the leaf can be affected—with up to 4,000 lesions on a single leaf.
#corn hybrid response to #tar spot- a means for reducing disease impact in subsequent seasons? https://t.co/BzGMPN0BCJ @ilcorn @ILextension @GMK_Agronomy @DTelenko @corndisease @badgercropdoc @JimDonnelly1 @ILPestSurvey @IllinoisCropSci pic.twitter.com/MLF4iOro8j— Nathan Kleczewski (@ILplantdoc) September 7, 2018
Hot off the press - a map of tar spot in Indiana. If your county is not red and you suspect you have it please get a sample to the @PurduePPDL or contact me. @badgercropdoc @ILplantdoc @PurdueExtension pic.twitter.com/hvT3Z8Cj2u— Darcy Telenko (@DTelenko) September 6, 2018
Hybrid resistance will be key to manage tar spot in 2019. Visit your closest hybrid trial now and observe which hybrids are still green. Don’t pick a brown hybrid with high levels of tar spot for 2019! https://t.co/8o6vDQFRc8— Damon L. Smith (@badgercropdoc) September 7, 2018
This is a good update on tar spot. Please keep on the lookout in Indiana. I was in some hard hit fields today. https://t.co/t30CWjdwD6— Darcy Telenko (@DTelenko) August 31, 2018