Corn Smut

Need to know:

• Smut galls are lumpy and mushroom-like with an off-white to grew color.
• The galls enlarge, filling with fine black spores.
• The spores can spread easily by wind and water.
• Prevent this problem by selecting corn hybrids.
• If older varieties are desired, the gardener should scout early and often, remove galls before they set spore, and avoid overhead irrigation.

Overview

Corn smut is caused by the fungus ustilago maydis. When severe, the infection can impact the number and quality of corn ears. While forms of resistance are bred into many modern corn cultivars, heirloom and landrace varieties are incredibly susceptible to the fungus. Therefore, any home gardener should be aware of the threat of corn smut when selecting varieties.

Symptoms of Corn Smut

Smut galls are lumpy and mushroom-like in appearance, with an off-white to grey color. 

Signs of Corn Smut

The galls begin as small growths on the leaves, stalks, and tassels of growing plants. They enlarge as they grow, filing with fine black spores (signs), called teliospores, that can be spread easily by wind and water.  In severe cases, entire ears will become infected with smut. Infected cobs will burst through the husks of the maturing ear, becoming very apparent. When entire ears are affected, yield can be dramatically reduced.

Management

The most effective management is prevention. Resistant cultivars are the best option for many growers and gardeners. The inclusion of smut resistance became common with the development of hybrid corn, which involves breeding together two very inbred lines. These inbred parents, being incredibly uniform in their traits like those relating to disease resistance, create offspring with all of the benefits and protection of the parents. Therefore, many modern varieties will not be severely impacted by the fungus.

However, if a gardener is interested in growing heirloom or older varieties, these cultivars will likely experience some degree of smut susceptibility. In many cases, susceptibility can be high. There are management techniques for reducing the damage:

1. Scout early and often
   • The best line of defense is catching smut before it becomes a major issue. 
   • Timing is crucial for removing galls by hand.
2. Fungicides have limited effectiveness and are therefore not recommended. There are no organic fungicide options for smut.
3. Remove galls before they set spore
   • This must be done early, before the galls burst open and spread their spores (this is why scouting is crucial). If the galls have mature spores inside, the black, sooty teliospores can spread easily and infect other plants.
   • When the galls are young, simply remove the smut by hand. Be cautious not to touch other corn plants after galls have been removed, as this could possibly spread the fungus to healthy plants. Take the galls out of the garden and throw them away or burn them – do not compost them.
4. Avoid overhead irrigation, since the splashing may spread the smut. Prolonged leaf wetness and humidity also contribute to this issue. Use drip irrigation if possible.
5. If you have smut in your garden, rotate your corn planting the following year
   • Corn smut spores can reside in your soil for many years.

Fungicide applications may be avoided by following good Integrated Pest Management practices like those listed in this encyclopedia article. Often, only preventative application is effective to manage plant diseases. If the problem requires a fungicide, state law requires the user to read and follow all labels accordingly. For more information, read Proper fungicide use.  

Fun Fact:

Corn smut is considered a delicacy in Mexican cuisine. Called huitlacoche or the Mexican truffle, the smut galls are collected when they are young and have a spongey texture. They are then typically sauteed and used in a variety of dishes like quesadillas or soups.

Prepared by: Emma Herrighty (Graduate student in Anthropology and Sustainable Agriculture) and Dr. Lina Rodriguez-Salamanca