Tom Volk's Fungus of the Month for March 1998

This month's fungus is Ustilago maydis, which causes corn smut.

If you're thinking ahead, you certainly realize that this month's page can be subtitled
Smuts on the internet

For the rest of my pages on fungi, please click

Ustilago maydis on corn I've come to the conclusion that there just isn't enough smut on the internet, so this month's fungus should help to rectify that situation. I've talked with people who have said "I don't understand how you can get so many hits on your pages-- there isn't even any smut there!" Well now there is.

[Please don't send me hate mail no matter what side of the "smut" issue you're on. For the record, though, I am a supporter of freedom of speech on the internet.]

Corn smut is caused by the basidiomycete fungus Ustilago maydis, sometimes called Ustilago zeae-maydis, which is pictured to the left in a photograph by Deon Nontelle of the Biology Department here at the UW-La Crosse. The common name "smut" comes from the appearance of the ripe galls containing thousands of dark sooty spores. If you're familiar with other basidiomycete fungi, such as mushrooms, puffballs and stinkhorns, you'll notice a distinct difference in this fungus-- there is no real fruiting body. The large black sac you see are the teliospores, the resting spores of the fungus that are produced to help the fungus overcome harsh conditions, such as drought and especially winter. The teliospores drop to the ground or are spread by the wind. In the spring they germinate to form basidia, which produce basidiospores. These basidiospores are the actual infective agent of the corn. Although many parts of the plant can be infected, most of the time the kernels of corn are the actual point of infection. The fungus grows through the kernel and causes two things to happen that cause massive smut galls to form-- Both of these, along with proliferation of the fungus itself, cause large galls to replace the kernels of corn, sucking the energy from the development of the other kernels, drastically decreasing the yield of the corn crop. The fungus can grow systemically (inside the plant), spreading to other ears of corn on the same plant as well as to many other parts of the plant, including the tassels (interfering with pollination) and the stem (interfering with transport of materials and growth).
Ustilago on cornHowever, smut is not all bad! Corn smut is considered a delicacy in Mexico! One has to eat it before the teliospores become ripe, when the insides are still moist and the gall is white to gray on the outside.. It's known as "huitlacoche" (sometimes spelled cuitlacoche) in the areas where it's eaten. Many Mexican farmers are delighted when this pathogen invades their crop, since they can receive a much higher price for the infected grains. Sometimes they even inoculate the fungus into the crop on purpose! There are many interesting dishes you can make with this fungus. I've tried it and find that it's really delicious. Since the gall is really a combination of cells from the fungus and the corn, much of the sweet flavor of the corn comes through when you cook it. For more information on corn smut as an edible fungus I suggest you check out One Person's Disease . . . Is Another Person's Delicacy, a very appropriate description of the contrasting sides of this disease. You can also check out this Mykoweb description of huitlacoche, which includes a recipe for huitlacoche soup. An authentic Mexican recipe (in Spanish!) for Huitlacoche con crema can be found at the web site of La Cocina Mexicana,a restaurant in Guadalajara, Mexico.
For more specific and practical information on Ustilago maydis as a pathogen of corn, I suggest you check out UC Pest Management Guidelines from the University of California at Davis. For more information about plant pathology in general I suggest you visit the home page of the American Phytopathological Society. Many plant pathologists are trying to STAMP OUT SMUT (mostly by breeding the corn for resistance) while others are trying to promote it for its delicious food value.
There are many other kinds of smut that infect different kinds of plants. All of them are pathogens to varying degrees, many on economically important members of the grass family. Laura Birch, an assistant Professor at Clarke College in Dubuque Iowa, wrote to me about another interesting smut, Microbotryum violaceum (formerly known as Ustilago violacea), which infects members of the pink family (Caryophyllaceae), including, but not limited to, Silene species. She has done some work on this fungus, and tells me that when M. violaceum infects the host plant Silene latifolia (=S. alba), female plants become male plants, and all host plants are effectively sterilized. The fungal spores fill the anthers. Therefore, this fungus is not transmitted via seeds. Insect pollinators are most likely the vectors for infection. Another fungus, Tilletia caries causes a "stinking smut disease" of wheat. And you thought you had problems...
Ustilago maydis has another interesting use in research projects. Although it usually exists as a mycelium in nature, in culture it grows as a single-celled yeast, so it can be manipulated exactly like Saccharomyces cerevisiae. A great deal is known about the physiology, genetics and molecular biology of this organism because of the ease with which it can be manipulated in the lab.

I hope you enjoyed learning something about smut. Every fungus has lots of interesting things about it. I encourage you to explore further and learn more about fungi. They're really interesting!

If you have anything to add, or if you have corrections or comments, please write to me at

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