Corn smut is a disease of maize caused by the pathogenic plant fungus Ustilago maydis. U. maydis causes smut disease on maize (Zea mays) and teosinte (Euchlena mexicana). Although it can infect any part of the plant it usually enters the ovaries and replaces the normal kernels of the cobs with large distorted tumors analogous to mushrooms. The black spores developing in these tumors give the cob a burned, scorched appearance. In fact, the name Ustilago comes from the Latin word ustilare (to burn).
Considered a pest in most of the United States, smut feeds off the corn plant and decreases the yield. Usually smut-infected crops are destroyed. However in Mexico corn smut is called huitlacoche (IPA ), and is considered a delicacy, even being preserved and sold for a higher price than corn. It is sometimes spelled cuitlacoche.
Huitlacoche grows best during times of drought in a 78°F to 93°F (25°C - 34°C) temperature range. Aztecs purposely innoculated corn with the spores by scratching their corn plants at the soil level with a knife—thereby allowing the water-borne spores easy entrance into the plant.
Life cycle
When grown in the lab on very simple media, it behaves like bakers yeast, forming single cells going by the name sporidia. These cells multiply by budding off daughter cells. When two compatible sporidia meet on the surface of the plant, they will switch to a different mode of growth. First, they'll send out conjugation tubes to find eachother, after which they fuse and make a hypha to enter the maize plant. Hyphae growing in the plant are dikaryotic ; they possess two haploid nuclei per hyphal compartment. In contrast to sporida, the dikaryotic phase of U. maydis requires infection of the plant in order to grow and differentiate and cannot be maintained in the laboratory.
Proliferation of the fungus inside the plant leads to disease symptoms as chlorosis, anthocyanin formation, reduced growth and the appearance of tumors harboring the developing teliospores (Banuett, 1995; Christensen, 1963).
Mature spores are released from the tumors and spread by rain and wind. Under appropriate conditions a probasidium is formed in which meiosis occurs. Resulting haploid nuclei migrate into elongated single cells. These cells detach from the probasidium; these are the sporidia, completing the life cycle.
Model system
The yeast-like growth is a big advantage for research, although its relevance in nature is unknown. The fungus is exceptionally well suited for genetic modification. This allows researchers to study the interaction between fungus and host with relative ease. The availability of the entire genome is another advantage of this fungus as model system.
Ustilago maydis is not only used to study plant disease. It has also been used to study recombination and DNA repair as well. That the function of the breast-cancer gene BRCA2 is now known is largely due to work with Ustilago maydis.
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