Tar spot of corn

Tar spot is caused by the fungus Phyllachora maydis. Tar spot appears as small, raised, black spots scattered across the leaf surface. These spots are ascomatum (fungal fruiting structures). If viewed under the microscope, hundreds of sausage-shaped asci (spore cases) filled with spores are visible. 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

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 United States.

Gallery Images: K. Wise and E. Zaworski