Recently, the Plant and Insect Diagnostic Clinic received samples of corn leaves with symptoms of tar spot from Jones County in eastern Iowa.
Tar spot was first reported in the United States in 2015 in Indiana, and was also confirmed later that season in Illinois. In 2016, the disease has again been found in Indiana and Illinois, with additional conformations in Michigan and Florida.
What are the symptoms of tar spot?
Tar spot is recognized as small, raised, black spots that occur scattered across the leaf surface (Fig. 1). These spots are fruiting structures, known as ascomata, of the fungus that causes tar spot, Phyllachora maydis. If a piece of the ascomatum is viewed under the microscope, hundreds of sausage-shaped asci filled with ascospores are visible (Fig. 2).
Figure 1. Close up of tar spot ascomata. Note the black spots are small and raised.
Figure 2. Cross section through the ascomatum of Phyllachora maydis showing the sausage-shaped asci filled with ascospores
Beware of “look-a-likes”
As with most diseases, tar spot does have “look-a-likes” – 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 (Fig. 3). Tar spots cannot be scraped off the leaf tissue.
Figure 3. Black teliospores of common rust pustules bursting through the epidermis of a corn leaf may be mistaken for tar spot.
What economic impact will tar spot have?
It is unlikely that tar spot will cause any yield loss in eastern Iowa where it was detected. Severity of the disease was very low and was detected late in grain fill. However, the fact that the disease is present in the U.S. for a second year suggests that we may see it again in the future.
Tar spot complex
In Mexico and Central America, where the disease is more common, tar spot alone does not cause economic damage. However, when tar spot is associated with another fungus, Monographella maydis, yield losses can occur. This disease complex is known as the tar spot complex. M. maydis has not been detected in the U.S.
For more information on tar spot, see this publication from Purdue Extension
Dr. Alison Robertson is an associate professor of plant pathology and microbiology. She provides extension education on the diagnosis and management of corn and soybean diseases. Her research interests include Pythium seedling disease of corn and soybean and Goss's wilt. Dr. Robertson receiv...
Edward R. Zaworski is a plant diagnostician in the Iowa State University Plant and Insect Diagnostic Clinic. He earned his master's degree in plant pathology in 2010, with a focus on field crop diseases.