Smuts are a large group of parasitic fungi that infect cereal crops and grasses, replacing healthy plant tissue with masses of dark, powdery spores. There are roughly 1,640 known species of smut fungi worldwide, and they attack some of the most economically important crops on the planet, including corn, wheat, barley, and oats. Despite their reputation as destructive pathogens, one type of smut is actually prized as a gourmet ingredient in Mexican cuisine.
How Smut Fungi Infect Plants
Smut fungi are biotrophic parasites, meaning they feed off living host tissue rather than killing cells outright. The infection starts when fungal spores land on a plant and germinate. The germinating spore produces tiny cells that are, on their own, completely harmless. Only when two of these cells fuse together does the fungus become capable of invading the plant. This fused form can penetrate the outer layer of the plant within 6 to 36 hours of landing on it.
Once inside, the fungus grows between and through plant cells without rupturing them. It creates an intimate contact zone with the host that allows it to siphon off nutrients and manipulate the plant’s own growth signals. The fungus essentially hijacks the plant from the inside, redirecting resources toward its own reproduction. Over roughly two weeks, it colonizes the tissue and begins producing masses of dark spores called teliospores. These spores replace kernels, flowers, or other plant structures, turning them into sooty black masses that eventually rupture and release spores into the air or soil to start the cycle again.
Common Types of Smut
Corn Smut
Corn smut, caused by the fungus Ustilago maydis, is one of the most recognizable smut diseases. It produces swollen, tumor-like galls on corn ears, stalks, leaves, and tassels. These galls start out light grey and firm, then darken to black as they mature and fill with spores. The fungus can switch between a harmless yeast-like form and a pathogenic filamentous form, which is why it can strike corn plants at virtually any stage of growth.
The economic damage is significant. A single gall on the upper stalk of a sweet corn plant can reduce the fresh weight of uninfected ears on that same plant by nearly 20%. Plants with multiple galls can lose about half their yield. Across the U.S. processed sweet corn industry, where Oregon and Washington alone account for 30% of production, even small infection rates add up to substantial losses.
Stinking Smut (Common Bunt)
Wheat is vulnerable to a smut known as common bunt or stinking smut, caused by Tilletia species. Instead of forming obvious tumors, this fungus replaces wheat kernels with “bunt balls,” which are sacs filled with dark teliospores. Fully infected wheat heads look noticeably different: shorter, with spikelets spread apart, giving the head a flattened, stilted appearance. Partially infected heads, though, can look completely normal from the outside.
The telltale sign is the smell. When bunt balls are crushed during harvest, they release trimethylamine, a volatile compound that gives off a strong rotten-fish odor. This smell contaminates the grain and makes it unsuitable for milling, so even low levels of infection can ruin an entire batch.
Loose Smut
Loose smut attacks barley, oats, and wheat. Instead of forming galls or hidden bunt balls, it replaces entire seed heads with olive-black masses of spores enclosed in a thin, fragile membrane. That membrane ruptures quickly, releasing airborne spores while the rest of the crop is still flowering. By the time healthy plants around it mature, a loose-smut-infected head is nothing but a bare stalk. The fungus survives on the surface of contaminated seed, so planting infected seed guarantees the disease will show up in the next crop.
Huitlacoche: The Smut You Eat
In most of the world, corn smut is a pest that farmers try to eliminate. In Mexico, it is a delicacy. Known as huitlacoche (roughly pronounced “weet-lah-KO-chay”), the young galls of Ustilago maydis are harvested from corn ears and cooked as a prized ingredient. At least 21 indigenous ethnic groups in Mexico have traditional names for it, reflecting centuries of culinary use.
The flavor profile is complex: earthy, slightly acidic, with bitter and umami notes. The most traditional preparation is a simple stir-fry with onion, garlic, chili pepper, and epazote (a pungent Mexican herb), then folded into quesadillas, tacos, or tamales. More recently, huitlacoche has been incorporated into soups, pastas, and pizzas, and international interest in the ingredient is growing. It holds high commercial value in Mexican domestic markets and is considered both a cultural icon and an alternative crop with real nutritional importance.
How Farmers Manage Smut
Because smut fungi are seedborne or soilborne depending on the species, prevention starts before planting. The single most important step is using certified, pathogen-free seed. Farm-saved seed from fields with any history of smut infection is a major source of reinfection.
Seed treatments with fungicides are the primary chemical control for loose and covered smut in grains. Foliar sprays applied after the crop is growing do not work because the fungus is already established inside the plant tissue by the time symptoms appear. Farmers are advised to rotate between different classes of fungicide to prevent resistance from building up.
Beyond seed treatment, crop rotation plays a key role. Resting fields from susceptible crops for two to three years reduces the spore load in the soil. Planting resistant varieties, where available, further limits infection risk. Equipment hygiene matters too, since spores can spread on combines, planters, and seed-handling equipment from one field or season to the next. For grass hay and seed operations, avoiding continuous cropping of the same grass species helps suppress volunteer plants that can harbor the fungus between seasons.
Are Smut Fungi Harmful to Humans?
Smut fungi are plant pathogens, not human pathogens. They do not infect people. The spores can be an airborne allergen for some individuals, similar to mold spores in general, but they pose no infection risk. Huitlacoche has been safely consumed in Mexico for centuries, and no toxic effects from eating properly prepared corn smut have been documented. The primary concern with smut is agricultural, not medical: it damages crops and contaminates grain with off-flavors and odors that make it unmarketable.