Corn smut is a common plant disease that can be startling for growers to discover due to its unusual appearance. This condition causes the corn plant to develop large, irregularly shaped growths, or galls, on various above-ground parts. These growths are a clear sign of infection and can vary significantly in size, sometimes becoming as large as a fist. The presence of these distinctive, tumor-like masses often raises questions about the health of the crop.
Identifying the Fungus and Symptoms
Corn smut is caused by the fungus Ustilago maydis, which specifically targets corn and its ancestor, teosinte. The most prominent symptom is the formation of fleshy, tumor-like galls that appear on the ears, stalks, tassels, and leaves of the corn plant. These galls are initially covered by a thin, silvery-white to greenish membrane.
As the galls mature, the interior fills with a mass of dark, powdery spores. The outer membrane eventually ruptures, releasing millions of black, dust-like teliospores into the environment. Galls on the leaves tend to remain small and blister-like, while those on the ear and stalk can become much larger. This black powder is the source of the name “smut.”
How Corn Smut Infects the Plant
The life cycle begins with teliospores, which are black, resting spores that can survive in the soil and on corn residue for several years. When conditions are favorable, particularly temperatures between 50 and 95°F and adequate moisture, these spores germinate. Germination leads to the formation of new spores called sporidia, which are carried by wind or splashed by water to new corn plants.
Infection occurs when these spores land on young, actively growing tissues, such as the silks of the ear or the whorl of the stalk. The fungus can also gain entry through fresh wounds, often created by insect feeding, hail damage, or mechanical injury. Once inside the plant, the fungus stimulates the host cells to multiply and enlarge, leading to the rapid development of galls within a few days.
The fungus grows within the corn plant as a dikaryotic mycelium, the pathogenic form that causes the galls. This growth phase involves fungal hyphae proliferating and fragmenting to form the diploid teliospores that will overwinter. Warm and wet weather conditions favor the spread of the infection, especially if combined with events that injure the plant tissue.
Management and Prevention Strategies
Since there is no effective chemical treatment once galls have formed, management focuses heavily on prevention and cultural practices. Selecting corn varieties with genetic resistance or tolerance to the disease is an important first step for growers in areas where smut is common.
Sanitation reduces the amount of inoculum present in the field for the following season. Infected galls should be carefully removed as soon as they are noticed and before their outer membrane breaks open to release the black spores. The removed galls must be destroyed, typically by deep burying or burning, and should not be composted or left in the field.
Growers should also minimize physical injury to the corn plants, as wounds serve as easy entry points for the fungal spores. This involves managing insect pests and avoiding mechanical damage from machinery. Balancing soil fertility is also beneficial, as high levels of nitrogen and organic matter can increase the susceptibility of the crop to infection.
Crop rotation helps reduce the concentration of overwintering teliospores in the soil, although the spores can survive for several years. Deep plowing of corn residue after harvest can bury the infected material, reducing the amount of inoculum near the soil surface. Systemic fungicides are not considered effective or practical for controlling corn smut in commercial corn.
Corn Smut as a Culinary Ingredient
Despite its designation as a disease, corn smut is considered a delicacy in Mexican cuisine, where it is known as huitlacoche or cuitlacoche. This “Mexican truffle” is intentionally cultivated and harvested before the galls rupture and release their spores.
When cooked, huitlacoche has a distinct earthy, smoky, and mushroom-like flavor with a soft, velvety texture. It is a prized ingredient used in dishes, including quesadillas, soups, and stews.
The fungus provides a nutritional benefit, being a source of essential amino acids, particularly lysine, which is often low in uninfected corn kernels. It also offers a higher protein content than normal corn, along with essential fatty acids and antioxidants. This high culinary value has led to commercial production efforts.