Tar spot of corn
Tar spot is caused by the fungus Phyllachora maydis, and can cause severe yield loss on susceptible hybrids when conditions are favorable for disease. Tar spot appears as small, raised, black spots scattered across the upper and lower leaf surfaces. These spots are ascomatum (fungal fruiting structures). If viewed under the microscope, hundreds of sausage-shaped asci (spore cases) filled with spores are visible. When severe, ascomatum can even appear on husks and leaf sheaths.
Tan to brown lesions with dark borders surrounding ascomatum can also develop. These are known as "fisheye" lesions. In Latin America, where tar spot is more common, fisheye lesions are associated with another fungus, Monographella maydis, that forms a disease complex with P. maydis known as the tar spot complex. M. maydis has not been detected in the United States.
At the end of the growing season, common and southern rust pustules can be mistaken for tar spot ascomatum as these rusts switch from producing orange-red spores (urediniospores) to black spores (teliospores). However, rust spores burst through the epidermis and the spores can be scraped away from the pustules with a fingernail while tar spots cannot be scraped off the leaf tissue. Characteristic tar spot symptoms and signs on corn leaf. Image: K. Wise
Fungal fruiting structures characteristic of tar spot. Image: E. Zaworski
The pathogen that causes tar spot overwinters on infested corn residue on the soil surface, and it is thought that high relative humidity and prolonged leaf wetness favor disease development.
Residue management, rotation, and avoiding susceptible hybrids may reduce tar spot development and severity. Some fungicides may also reduce tar spot, although little data exists regarding application timing for efficacy and economic response.