Corn smut. Corn showing damage caused by the corn smut fungus, Ustilago maydis. Corn smut occurs wherever corn is grown throughout the world. The fungus damages plants and reduces yields by forming galls on the ears, stalks and leaves. Galls here (grey) can be seen to have greatly damaged the ear of corn (yellow). When the galls mature their interior turns into a mass of powdery, dark, olive-brown spores. The grey membrane of the gall then ruptures (as seen at upper left) releasing the millions of sooty teliospores (brown) into the atmosphere. These spores overwinter in the soil before germinating.