||Caption: Ustilago maydis is a pathogenic fungus that causes corn (maize) smut disease. Shown here is the budding yeast stage (laboratory grown) and its mucilaginous matrix. When grown in the lab on very simple media, Ustilago maydis behaves like baker's yeast, forming single cells, commonly named sporidia. These yeast cells multiply by budding off daughter cells (shown here). When two compatible sporidia meet on the surface of the plant, they switch to a different mode (phase) of growth. First, they send out conjugation tubes to find each other, after which they fuse and make a hypha to enter the maize plant. Hyphae growing in the plant are dikaryotic and this phase of requires infection of the plant in order to grow and differentiate, thus it cannot be maintained in the laboratory. The fungus damages the corn plants and reduces yields by forming galls on the ears, stalks and leaves. When the galls mature (dikaryotic phase) their interior turns into a mass of powdery, dark, olive-brown spores. The grey membrane of the gall then ruptures releasing the millions of teliospores. These spores overwinter in the soil before germinating the next year on a new corn crop. Corn smut occurs wherever corn is grown in the world.