Mushrooms, Plant Diseases

Corn Smut: Bad For Corn, But Delicious


Corn smut

The first time I tasted corn smut was in a food hall in Mexico City while visiting some friends there. Before making our orders, my friends urged me to try huitlacoche (pronounced weet·luh·ko·chay), a delicacy in Mexico and a unique ingredient that I wouldn’t be able to find easily back home. As a lover of edible fungi, I eagerly dug in. To my delight, I enjoyed the earthy flavor and the delicate texture of huitlacoche and have been on the lookout for them in the U.S. 

We have all heard of the old adage that when life gives you lemons, make lemonade.  For the Aztecs and other indigenous people of North America, they discovered the fungus Ustilago maydis on young maize, which stunted the growth of the corn kernels and turned them into grayish blue galls. This fungal growth turned out to be edible and highly nutritious, containing more protein than unaffected corn, and even has an added amino acid (lysine) that is deficient in regular corn protein. Corn smut, sometimes referred to as Mexican truffle or Mexican caviar, is a great meat substitution and has become more available and appreciated outside of Mexico. 

Ustilago maydis thrives under a specific set of weather conditions and most gardeners do not set out to cultivate this fungus specifically when growing sweet corn. The fact that it is edible means that the whole harvest is not lost if you do find your corn afflicted.

What Is Corn Smut?

Corn smut
Corn smut is bad news for corn, but not necessarily for us. Source: Björn S.

Corn smut or common smut is the result of the fungal pathogen Ustilago maydis, a member of the smut fungi phylum Basidiomycota. This group of plant pathogens has a very narrow range of hosts and must depend on their host plant to survive. U. maydis is only able to grow on the corn plant (Zea mays) and its non-domesticated ancestor Zea mays subsp. parviglumis. This group of smut fungi uniquely targets the immune system of their hosts and leverages the hosts’ metabolism to benefit their own reproduction and growth. This plant pathogen is found throughout the world and causes significant economic losses to farmers who are not in the business of selling corn smut-infected plants. 

Life Cycle Of Ustilago maydis

Ustilago maydis has a complex disease cycle that involves a saprophytic stage when the fungus uptakes nutrients and an invasive stage when it hijacks host plants. The fungus is dimorphic and first develops as a saprophytic haploid sporidium containing one unpaired chromosome. Sexual reproduction of this fungus occurs when two haploid cells fuse into a dikaryon which has a special infection structure to invade host tissue. The fungus then proliferates and differentiates within the tumors or galls of its host plant and produces black diploid teliospores.  When the galls rupture, they release teliospores into the wind which can transport them over long distances. These spores can overwinter in crop debris and soil.  Under favorable conditions of dry weather followed by wet weather during the corn growing season, they germinate and undergo meiosis to produce the haploid phase and start the cycle anew. 

Symptoms Of Corn Smut

The fungus affects the above-ground parts of corn plants. Galls begin as small swellings and whitish-gray irregular kernels. These kernels will then increase in size and can grow up to 4-5 inches in diameter. The galls will gradually turn black as they develop spores before eventually rupturing and releasing spores into the wind to infect neighboring plants. In fact, the scientific name, “ustilago” derives from the Latin word ustilare, which means to burn, because of their sooty teliospores. The infection is not systemic, which means that it’s not throughout the whole plant, but only in places where the fungus has attached. This is why you may see corn ears with a mix of healthy and infected kernels. You can also sometimes find corn smut galls on corn leaves, although it is not recommended to eat these. 

Controlling And Preventing Corn Smut

Ustilago maydis under a microscope
Ustilago maydis viewed under a microscope. Source: Björn S.

Corn smut is more prevalent if corn pollination occurs during hot and dry weather followed by heavy rains and wet weather. Additionally, excess nitrogen in the soil can also increase the pathogenicity of corn smut. There are no fungicides that can prevent corn smut. The spores of the common smut fungus overwinter in the soil and can cause infections even after several years, poised for the right weather conditions. Crop rotation can help to disrupt their life cycle since this fungus can only survive on corn hosts. Legumes plants are a good option for crop rotation between seasons of corn. 

An IPM control methodology can be used to maintain a sanitary garden or field. Take care to remove corn debris and clean cultivation tools and machinery. Typically, the common smut can affect 1-5% of field corn. Avoid mechanical damage to corn plants and inspect them for damages which are wounds that make the plant more vulnerable to infection of all kinds including corn smut. A few infected plants in a cornfield should not be a major issue if they are discovered before the corn smut matures and has a chance to release airborne spores. 

You can also try to cultivate huitlacoche through artificial inoculation prior to corn pollination. Soak the corn silks in a solution with common smut spores mixed with water. If you were able to successfully inoculate your corn, you should see galls form within two weeks. Harvest the galls after 16-18 days from inoculation for peak flavor. Naturally occurring galls will be much larger than cultivated galls because only a few kernels are infected as opposed to the inoculation method which induces tumors in every kernel. 

To harvest huitlacoche, look for young plants that have immature galls. Pluck or cut away the mushroom-like galls from the corn cob for immediate use. These galls are highly perishable and do not keep well in the refrigerator for more than a few days before the galls burst. Fresh huitlacoche is sold for $15-20 per pound in the U.S. because they are rare and difficult to transport. Some CSAs and farmers’ markets carry them on occasion during the corn growing season. A common method of preserving them long-term is to freeze the galls, although there will be some changes to the texture. Many restaurant and home chefs can now purchase frozen huitlacoche from online vendors.

Frequently Asked Questions

Huitlacoche
Known as “huitlacoche” in Mexico, corn smut is an edible fungus. Source: Wendell Smith

Q: Is corn fungus safe to eat?

A: Immature corn smut galls resulting from the fungus Ustilago maydis or common smut is safe to eat and considered a delicacy. Although leaf galls may be present on a plant, leaf galls are not recommended for eating. Corn can be affected by multiple smut diseases including another pathogen called head smut. This smut disease causes a systemic infection throughout the plant and is not edible. 

Q: Is corn smut the same as huitlacoche?

A: Yes, corn smut is another name for huitlacoche, an abnormal growth on corn plants caused by the fungus, Ustilago maydis.

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