wheat field

Special Edition – Effects of Smut in Corn

By Dr. K.E. Lanka, Ph.D., P.A.S.

Smut describes several diseases in grains such as corn, sorghum and small grains that are caused by fungi. In this article, only smut diseases in corn will be discussed. The most prevalent type of smut in corn is “common smut,” known as Ustilago maydis or the older name Ustilago zeae. The second and less prevalent type of smut is “head smut,” which is known by the scientific names, Sphacelotheca reiliana or Sporisorium holci-sorghi.

Common smut forms thick, fleshy, galls (mushroom-shaped structures) on the ear, tassel, stalk, and leaves of a corn plant; however, the kernels on the ears are the most likely sites for smut infections. Common smut galls on the roots are not very common. Smut on the ears and tassels are of greatest concern, as they are the most destructive to the corn crop, with negative economic effects to the farmer. The spores are infectious and can affect different parts of the plant at the same time.
At right, Common smut, Ustilago maydis (Photo by Tamra Jackson-Ziems, Ph.D., UNL Extension Plant Pathologist)

Head Smut_June2016 Special IssueBy contrast, the head smut affects the plant systemically. The head smut structures are thin-walled and burst early in tassel and ear development to release dark brown to black colored spores. Infected plants have vascular, finger-like projections, surrounded by the dark spores. This is a characteristic that distinguishes head smut from the common smut. It is neither observable nor obvious until the tassels and ears appear on the plant. Unlike common smut, it rarely affects the stalk or leaves.
At left, Head smut, Sphacelotheca reiliana (Photo by Tamra Jackson-Ziems, Ph.D., UNL Extension Plant Pathologist)

Spores of both common and head smut can survive in the soil for several years. These are called teliospores, and the duration of the survival is dependent on environmental conditions. They can survive winter conditions, as well as warm, dry environments. Teliospores may survive locally, or they may be blown long distances by the wind. They may also be transported on grain from one region to another. Teliospores of corn smut will germinate in moist air and form buds like a yeast cell. The new spores, called sporidia, then continue the germination process in water, such as dew, irrigation or rainwater that collects on the plant leaves, ears, tassels and other structures of the corn plant. These lead to infections that are visible within about 10 days.

Stressed plants are more susceptible to smut infections than healthy plants. Damage to the plants, such as from wind, hail, corn borers, and insect infestation, as well as open wounds in the plant, allow the sporidia to enter and infect them. Other causes of damage to the plants are from farm machinery which can scratch or damage them and open the tissues for infection. Fields that are not irrigated in a hot, dry summer may also be more likely to be infected, due to heat stress on the plant. During summers that average hotter than average temperatures, smut infections will likely be more than normal. Less infection will be seen during cool summers, even though soil moisture is average or higher than normal.

An imbalance in soil fertility can also lead to an increased incidence of corn smut. Soils that have had animal manure applied may be high in nitrogen and organic matter, and these conditions can lead to high levels of smut. However, on the contrary, plants that are stressed due to low soil fertility may also lead to more smut than fields with balanced nutrition.Common Corn Smut_June2016 Special Issue

Corn smut does not usually have a negative economic impact on the value of corn unless infection of the crop is severe. Sweet corn or popcorn is somewhat more susceptible to the fungi and can suffer from loss of value when there is a high level of infection. Losses are greatest when the ears become infected.

Properly prepared corn silage helps to dilute the concentration of the smut spores. The acids produced by the ensiling process kill the spores in the infected plants. However, spores in grain may pass through an animal and remain viable in the manure. When this manure is spread on fields, the spores can germinate and infect plants for several years.

In heavily infested grains, livestock may find the smut spores irritating to the nasal passages. Otherwise, there are no known direct hazards from feeding the grain, based on scientific literature. Unlike molds, smut does not produce mycotoxins that can adversely affect the livestock. However, when harvesting corn from fields that are heavily infested with smut, the dark cloud can be irritating to combine operators and workers who are exposed to the high concentration of spores. In such cases, it is advisable to wear a face mask or other protection for nasal passages and respiratory system.

In a recent scientific study comparing corn silages from smut-infested (10 to 15% of the plants) and non-infected plants, protein and mineral compositions were not different. The IVDMD of the corn silages decreased (P<0.05) as smut infestation increased. However, in vitro NDF digestion increased (P<0.05). When a choice of the two silages was offered to wethers, they ate more of the smut-infested than the non-infected corn silage (P<0.05). Thus, there was no negative effect on palatability from smut fed to wethers.

There is no known scientific literature that indicates a negative effect of smut-infested forage on production in animals. Likewise, smut infestation has not been shown to cause breeding problems in livestock.

Also, in humans, common smut does not cause adverse health effects. In some cultures, immature smut galls, which are closely related to other fungi such as in mushrooms, are eaten as a delicacy. Young corn plants are purposely scarred to increase infection by the common smut spores. The galls are then harvested while they are whitish-gray before they turn black. When harvested from sweet corn, the smut fungus has a sweet flavor from the corn that it has been feeding on. After harvesting the galls, they are then cooked until they turn black and have an earthy flavor. In Mexico, this delicacy is called huitlacoche, cuitlacoche or maize truffles. In the U.S., Mexican restaurants serve it as corn truffle.

There are few effective management strategies. Planting resistant hybrids and attention to not wounding plants can help. Some methods to control infection by smut fungi include:
• Maintain appropriate soil fertility.
• Avoid injury to roots, stalks, and leaves when cultivating, spraying and fertilizing the fields.
• Apply insecticides to reduce damage from insects. Corn borers can best be controlled by spraying when tassels first appear.
• Plant insect-resistant hybrids.
• Plant smut resistant hybrids.
• Plant early when the soil temperature is still cool.
• Rotate corn with other crops, such as soybeans.

Agri-King, Inc., is a leader and innovator in animal nutrition. Agri-King rations are balanced to meet the nutrient needs of the animals that are fed by clients. High quality forages grown on the farm reduce costs needed for ingredients purchased from outside sources. It is recommended that Silo-King be used in the treatment of corn silage, other wet forages and high moisture grains for better digestion and utilization in cattle. For more information about Agri-King nutrition, contact a representative near you or visit the website at www.agriking.com. AK

References:
1. Babadoost, M. 2002. Corn Smuts. Univ of Illinois Integrated Pest Management publication RPD No. 203. Univ of Illinois Extension, Urbana-Champaign, IL.
2. Cole, N. A., C. M. Rush, and L. W. Greene. 2001. Influence of Corn Smut on the Palatability and Digestibility of Corn Silage. The Professional Animal Scientist 17:287.
3. Draper, M. A. 2004. Common Corn Smut. South Dakota State Univ Extension publication FS 918. Cooperative Extension Service. SDSU, Brookings, SD.
4. Jackson-Ziems, T. A. 2014. Smut Diseases of Corn. NebGuide Extension publication G2223. Institute of Agriculture and Natural Resources. Univ of Nebraska, Lincoln, NE.