Corn Smut

Ustilago maydis, a fungus

Corn smut [Credit: Eric Shmelz]

Corn smut [Credit: Kai Hirdes]

Host Plants:

On Crops: All types of corn

Where Found:

Mexico and southern North America

Description:

In summer when corn is almost ready to pick, kernels become disfigured into knobby blobs marbled with gray and black. Sometimes other plant parts show these growths as well, but distorted kernels that pop out through the husk are most common. Outbreaks occur where temperatures range between 80 and 92F (27-33C) in midsummer. Spores carried by wind cause the infection to develop long before the distorted kernels are seen.

Damage:

Infected ears are considered inedible in North America. In Mexico, they are a delicacy tasting something like mushrooms, called huitlacoche.

Preventing Problems:

Rotate sweet corn, because corn smut fungi can persist in soil for three years. Sometimes spores enter ears through feeding holes made by insects, so growing varieties with tight, thick husks may be a useful strategy. Early sweet corn has fewer problems with smut compared to varieties that mature later in the season. When horses or cows eat smutty corn ears, the fungi can pass through their systems in their manure. In high risk areas like the southeastern US, use fertilizers other than manure to grow sweet corn.

Managing Outbreaks:

As soon as you see them, pick off infected ears and dispose of them in the trash or an active compost pile. Or, harvest the ears and store them in the refrigerator for use in Mexican recipes.

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