Open Your Eyes To Common Smut In Sweet Corn

Common smut of corn
Photo by David Langston

Disease Specs

Common smut, caused by the fungus Ustilago maydis, was probably present when Europeans first came to America and is now present in nearly all countries where corn is produced. Losses to smut are generally light, but may be important in some situations, particularly in sweet corn.

Identification

Infection by the smut fungus results in galls, which can form on any part of the plant above the soil. Most commonly, galls occur on stems near nodes. Large galls may appear on stalks at the nodes, on ears, and rarely on tassels. Leaf infections may result in small inconspicuous galls. Sometimes, galls form on leaves, leaf axils, and brace roots above the ground.

Initially, a gall is a compact mass of white, fungal hyphae that gradually becomes transformed into a mass of dark teliospores enveloped by a whitish-colored covering (aka, the peridium).

On ears or stalks, the galls expand rapidly and are covered with a thin greenish-white or silvery-white tissue. As the galls mature, the covering ruptures exposing masses of black spores within. Individual galls on stalks may be up to six inches in diameter. On infected ears, galls originating from individual infected kernels may combine to form the compound gall that replaces most of the ear.

In Mexico, immature smut galls are consumed as an edible delicacy known as cuitlacoche, and sweet corn smut galls have become a high-value crop for some growers who sell them to Mexican restaurants.

Survival And Spread

Hot dry weather is favorable for the growth of the fungus and the dissemination of wind-blown spores. The spores, however, require free moisture collected in the silk, leaf blades, and other parts of the corn for germination and growth for infection to occur. Smut spores may be blown long distances by the wind.

Plants may be infected at any time in the early stages of development, but grows less susceptible after the formation of the ear. Infection occurs when the teliospore germinates and forms a germ tube-like structure (promycelium), which penetrates the tissue or by the formation of a similar structure formed by the mating of two opposite mating types.

Warm temperatures from 79°F to 93°F are conducive for infection. Wounds caused by insects, hail, fertilizers, cultivation, blowing sand, or other causes can increase the severity of smut. Young and meristematic tissues are most susceptible to infection.

The black spores (teliospores) produced in smut galls can survive in the soil and serve as inoculum.

Management Methods

Seed treatment is of no value in controlling smut. There are no chemical controls available. Cultural practices, such as destruction of crop debris and crop rotation, are of limited value. Physical damage to corn plants should be minimized as this might promote infection.

Smut is more likely to occur in field corn than in sweet corn. Some varieties of field corn are highly susceptible. Control of smut in sweet corn is best achieved by the use of resistant varieties.

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