Corn smut is a fungal disease that causes swollen, tumor-like growths called galls on ears of corn, stalks, tassels, and leaves. The fungus responsible, Ustilago maydis, occurs worldwide wherever corn is grown. While most American farmers treat it as a destructive crop pest, the same infected kernels are a prized culinary ingredient in Mexico, where they’re known as huitlacoche and can sell for as much as $40 a pound.
How the Fungus Infects Corn
Corn smut begins when two compatible fungal spores land on the surface of a corn plant and mate. They recognize each other through a chemical signaling system and fuse together, forming a new structure called a dikaryotic filament. This filament is the infectious form of the fungus. It grows along the plant surface until it detects the right signals to form specialized penetration structures called appressoria, which allow it to break through the outer layer of the plant.
Once inside, the fungus grows between and within plant cells, pushing into the host’s cell membranes and establishing an intimate feeding zone. As the infection progresses, the fungus triggers irregular, uncontrolled division of the corn’s own cells, producing the characteristic bulging galls. These galls can appear on any aboveground part of the plant but are most dramatic on the ear, where they replace individual kernels and can swell to several inches across.
Young galls are covered in a greenish-white, papery skin. As they mature, the interior darkens into masses of powdery, olive-brown to black spores. When the gall eventually ruptures, it releases millions of these spores into the air, spreading the fungus to other plants. The infection is opportunistic: wounds from hail, insects, or mechanical damage give the fungus easy entry points, and warm, dry conditions followed by sudden rain favor its spread.
Crop Damage and Yield Loss
For corn growers, smut is a genuine economic problem. Worldwide yield losses in field corn typically range from a trace to about 10%, but individual infected plants suffer far more. A plant with a single gall loses roughly 25% of its yield. Plants with multiple galls lose about 50%.
The location and size of the gall matter too. A single gall on the upper stalk reduces ear weight by nearly 20%, while one near the base of the stalk causes only about a 7% loss. Larger galls are more damaging: galls over four inches in diameter cut ear weight by close to 20%, compared to around 6% for galls under two inches.
Management options are limited. Fungicides are not effective against corn smut. Growers rely instead on planting resistant cultivars, rotating crops, minimizing mechanical injury during cultivation, and controlling insect pests that create entry wounds. Removing galls before they mature and release spores can reduce spread in small plantings, though this is impractical at commercial scale.
Huitlacoche: A Mexican Delicacy
In Mexico, the same fungal galls that American farmers discard are harvested, cooked, and eaten. Huitlacoche (sometimes spelled cuitlacoche) has been part of Mexican cuisine for centuries. The earliest documented reference appears in the Florentine Codex from the mid-1500s, which includes both an illustration of the infected ear and a description in Nahuatl, the Aztec language. The name itself comes from Nahuatl words meaning, roughly, “pig’s excrement,” a reference to the gall’s appearance rather than its taste.
At least 21 ethnic groups in Mexico consume huitlacoche, and it holds a place in traditional medicine as well, where it has been used to treat dozens of ailments. Today it shows up in quesadillas, tacos, enchiladas, sopes, and tlacoyos, and has been incorporated into more modern dishes like soups, pasta, pizza, and baked goods. Its nickname, “the Mexican truffle,” reflects both its earthy complexity and its price point.
What It Tastes Like
Huitlacoche has a layered flavor that people describe as earthy, slightly acidic, mildly bitter, and rich with umami. The taste is unlike any common mushroom. Its distinctive aroma comes largely from a collection of aldehydes, compounds produced when the fungus breaks down fatty acids like oleic and linoleic acid during infection. Vanillin, the same compound that gives vanilla its scent, is one of the key aroma contributors. Another important compound, sotolon, adds a sweet, caramel-like note often associated with aged wines and maple syrup.
The galls are harvested while still young and moist, before the interior dries into powdery spores. At this stage, the flesh is soft, dark gray to black, and has an almost creamy texture when cooked. Overcooking flattens the flavor, so it’s typically sautéed briefly with onions, garlic, and chili peppers before being folded into a dish.
Is It Safe to Eat?
Corn smut does not produce the kinds of dangerous toxins found in some other crop fungi. Unlike molds that contaminate grain with harmful compounds, Ustilago maydis has been consumed safely for centuries across Mexico. The fungus does produce a compound called ustilagic acid, a type of glycolipid with antimicrobial properties, but it breaks down when taken orally and poses no practical threat to humans eating cooked huitlacoche.
The long history of safe consumption across multiple cultures is the strongest evidence of its safety. Fresh huitlacoche is perishable, though, and should be refrigerated and used within a few days of harvest, just like any fresh mushroom. Canned versions are widely available in Mexican grocery stores and online, making it accessible outside the growing season.
How to Identify It in the Field
Corn smut is one of the easiest plant diseases to recognize. The galls are unmistakable: pale, bulging growths that balloon out from the ear, replacing normal kernels. On tassels and leaves, galls tend to be much smaller. The greenish-white outer membrane is smooth and firm when fresh. As the gall ages, it softens and eventually splits open to reveal the dark, sooty spore mass inside.
If you’re a gardener who finds corn smut on your plants, you have two choices. You can remove the galls before they open to limit spread, or you can harvest them young for the kitchen. The ideal harvest window is when the galls are still firm and silvery-gray on the outside, with a moist, dark interior. Once they’ve dried to a powdery black, they’re past their culinary prime.