Recently, the Plant and Insect Diagnostic Clinic at Iowa State University received samples of corn leaves with symptoms of tar spot disease from Jones County in eastern Iowa. ISU Extension plant pathologist Alison Robertson provides the following information about this new disease in 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 confirmations 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).
ok-a-likes” that can cause confusion
As with most diseases, tar spot does have “look-a-like” diseases: common rust and southern rust. At the end of the corn 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, 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.
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.
What is the 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. Note that M. maydis has not been detected in the United States. For more information on tar spot, see these publications from Purdue University Extension: