Cases of tar spot in corn have been reported over recent weeks in 12 Iowa counties. Iowa State University Extension and Outreach plant pathologists have been able to confirm the presence of tar spot in four counties via the Iowa State University Plant and Insect Diagnostic Clinic. Agronomists and collaborators have confirmed the remaining cases throughout the state.
The current counties with confirmed tar spot presence include: Jones, Jackson, Johnson, Muscatine, Fayette, Clayton, Black Hawk, Buchanan, Delaware, Dubuque, Clinton and Scott counties.
Knowledge of yield impacts of tar spot in corn is limited. In Mexico and Latin America, tar spot, caused by the fungus Phyllachora maydis, does not affect yield. However, if P. maydis co-infects with another fungus, Monographella maydis, causing tar spot complex, then substantial yield loss may occur. To date, M. maydis has not been reported in the continental U.S.
University of Wisconsin plant pathologists have suggested however that P. maydis is capable of forming a complex with other organisms present in the Midwest, and the impacts of these complexes are currently unknown.
Tar spot in corn was first confirmed in the U.S. in 2015 in Illinois and Indiana. The disease was first reported in Eastern Iowa in 2016, the same year that it was detected in Michigan and Florida. Since then, the disease has been reported each growing season, which suggests that the fungus is overwintering in the Midwest. Reports of tar spot in eastern Iowa in 2018 have been received late in grain fill, and severity in most fields is low.
Tar spot in corn is recognized as small, raised, black-irregular-shaped spots scattered across the leaf surface. These spots are fruiting structures of the fungus, known as ascomata P. maydis.
If a small section of the ascomatum is viewed under a microscope, hundreds of sausage-shaped asci filled with ascospores are visible (Figure 3).
As with most diseases, tar spot does have “look-a-likes” – namely, common and southern rust. At the end of the growing season, both rust fungi switch from producing orange-red uredinospores, to black teliospores. Rust pustules filled with teliospores can be mistaken for tar spot ascomata. Remember that rust spores burst through the epidermis and the spores can be scraped away from the pustules with a fingernail. Tar spots cannot be scraped off the leaf tissue.
Researchers postulate that the pathogen is spread via wind and rain water. It has been proposed that spores of the pathogen arrived in the U.S. in a storm that originated in Mexico and Latin America.
If you have observed corn with tar spot symptoms, please notify an Iowa State University Extension and Outreach plant pathologist (Alison Robertson and/or Ed Zaworski) or tweet at @isu_ipm, with a photo (if possible) and the name of the county in which the disease was found, so that we may continue to add to our database and keep track of the disease in Iowa.