Common smut on maize
Maize ear infected with common smut (Ustilago maydis). The characteristic galls form on most aboveground parts of the maize plant, including ears, stalks, tassels and leaves; on the ears they replace individual kernels. Galls are initially greenish- or grayish-white with a shiny periderm. As they mature they become dark gray and contain masses of black spores (teliospores). Eventually the periderm dries and disintegrates, releasing the spores. The galls that form at the tip of the ear are the most distinctive and can reach up to 15cm in diameter. In Mexico and Central America, galls on the ear are consumed as a delicacy known as "huitlacoche".
For more information, see CIMMYT's Maize Doctor: maizedoctor.cimmyt.org/index.php?option=com_content&t....
Photo credit: CIMMYT.