Corn smut

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. These tumors, or "galls", are made up of much-enlarged cells of the infected plant, fungal threads, and blue-black spores. The spores 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, sometimes spelled cuitlacoche), an Aztec word reportedly meaning raven's excrement. It is considered a delicacy, even being preserved and sold for a higher price than corn. For culinary use, the galls are harvested while still immature &mdash; fully mature galls are dry and almost entirely spore-filled. The immature galls, gathered two to three weeks after an ear of corn is infected, still retain moisture and, when cooked, have a flavor described as mushroom-like, sweet, savory, woody, and earthy. Flavor compounds include sotolon and vanillin, as well as the sugar glucose.

The fungus has had difficulty entering into the American and European diets as most farmers see it as blight, despite attempts by government and high profile chefs. In the mid-1990s and due to demand created by high-end restaurants, Pennsylvania and Florida farms were allowed by the United States Department of Agriculture (USDA) to intentionally infect corn with huitlacoche. Most observers consider the program to have had little impact, although the initiative is still in progress. Regardless, the cursory show of interest is significant because the USDA has spent a considerable amount of time and money trying to eradicate huitlacoche in the United States. Moreover, in 1989 the James Beard Foundation held a high-profile huitlacoche dinner. This dinner famously tried to get Americans to eat more of it by renaming it the Mexican truffle.

Huitlacoche grows best during times of drought in a 78°F to 93°F (25°C–34°C) temperature range. Aztecs purposely inoculated corn with the spores by scratching their corn plants at the soil level with a knife&mdash;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 baker's 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 switch to a different mode 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; 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.

Uses
Native Americans of the American Southwest, including the Zuni tribe, have used corn smut to induce labor. It has similar medicinal effects to ergot, but weaker, due to the presence of the chemical ustilagine.

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, DNA repair and the role of the cytoskeleton in polarized growth as well. That the function of the breast-cancer gene BRCA2 is now known is largely due to work with Ustilago maydis.

Availability
Huitlacoche can be bought as a canned good in some markets and over the net. Some farmers markets and organic growers are endeavoring to bring fresh huitlacoche to their customers and local food service trade. The Troy Community Farm of Madison, Wisconsin started a project in 2003 to grow corn and produce huitlacoche to make available in the Madison area and now sell it directly to the public from their farm stand once a week.