How to Identify, Treat, and Prevent Corn Smut

A Corn Fungus That You Can Manage Through Prevention and Control

Corn smut (Ustilago maydis)

Björn S... / Flickr / CC by 2.0

When abnormal growth appears on the corn in your garden, it’s probably corn smut. Farmers also call it “devil’s corn” which gives you an idea how devastating this fungus is for the corn harvest. Corn cobs affected by corn smut are visually unappealing and not something that you want to serve at your summer barbecue. The corn is also unsuitable for freezing and canning.

For lovers of Mexican cuisine though, there is a bright side to it, too: the mushroom-like growth on the corn is viewed as a delicacy called cuitlacoche and also referred to as Mexican truffle.

Symptoms of Corn Smut

Any type of corn can be affected by corn smut, both field corn grown for animal fodder and popcorn, sweet corn and maize for human consumption.

Corn smut is an abnormal growth that can appear anywhere on the above-ground parts of the corn plant – leaves, stalk, husks, tassels, ears and corn kernels – and any time during the growing season. All young plant parts are especially susceptible.

Corn smut disease (Ustilago maydis) on corn stalk
saraTM / Getty Images

At the beginning, when the abnormal growth (galls) are small, they are covered with a white or silvery film. All galls except for the ones on leaves grow significantly up to four to five inches in diameter into a fleshy, smooth, mushroom-like body filled with masses of spores. Corn smut galls on leaves, on the other hand, don’t grow much, they stay small and hard.

Once the galls have matured, their outside hardens and turns brittle, and the spores inside turn into a black powder. Eventually the galls burst and release the powdery spores into the environment. But hopefully you will have caught the disease before that point to prevent it from spreading.

Corn smut (Ustilago maydis) on maize ear
Carmen Hauser / Getty Images

Causes of Corn Smut

Corn smut is caused by a fungus, Ustilago maydis. Its spores (the microscopic biological particles through which fungi are reproduced) can survive for several years in the soil and in corn debris.

The spores are spread in different ways—by wind, splashing water (rain or irrigation), or manure of animals that have eaten corn with the fungus.

There are several possible points of entry where the fungus attacks the corn. In corn ears, it’s typically the silks, But the fungus can virtually enter plants anywhere where the plant has a wound from insects feeding, hail or wind damage, or mechanical damage from weeding.

Weather conditions, both too much precipitation or too little, can also create favorable conditions for spreading corn smut. Temperatures between 79 and 73 degrees F and not enough rain, or rainy, wet weather can both lead to the spreading of corn smut.

Corn smut can also be a connected to the soil condition. Soils low in fertilizer as well as soils that contain excessive nitrogen have both been reported to increase the vulnerability of corn plants to the fungus.

And, finally, poor pollination due to the absence of pollinating insects or due to dry weather can also create conditions favorable to corn smut.

Corn smut caused by caused by the pathogenic fungus Ustilago maydis
Cylonphoto / Getty Images

How to Prevent and Treat Corn Smut

Prevention of corn smut starts with the selection of resistant corn varieties. While there is no variety that is fully resistant to the disease, some corn hybrids are more resistant than others. White Sugary Enhancer (SE) cultivars are reported to be more resistant than White Supersweet or Bicolor Supersweet varieties. In seed catalogs, sugary-enhanced hybrid corn varieties are usually identified by the acronym “SE”.

Following the rules of crop rotation is another way to prevent corn smut. Don’t plant corn in the same garden bed for two years in a row, or, if feasible, wait three or more years to replant it. If there are any corn smut spores left in the soil, they will die in the soil by the time you plant your next crop of corn.

Make sure that your soil is adequately fertilized, and avoid excess nitrogen. Do a soil test to determine which nutrients your soil is lacking.

When you weed around your corn plants, apply extra care not to injure them, as even a small nick can be an entryway for the spores.

If your corn has corn smut, there is no fungicide to control it. When you notice galls on your corn, remove them promptly and dispose of them in a safe way by burning them or throwing them in the garbage. Any pruners, knife or other tools that you used to remove the galls must be disinfected with a 10 percent bleach solution (one part bleach to nine parts water) between the cuts to prevent the fungus from spreading from one location to another.

When you put your garden to rest in the fall, thoroughly remove any corn debris.

Preventing corn smut from spreading might involve talking to your neighbors about corn smut on their corn patch. If galls are not removed from a plant early, it does not only mean the loss of the crop, but it will also likely lead to the continuation of the disease next year. Because once a mature gall bursts, the spores spread beyond control and will overwinter and may attack your corn plants next year.