Common smut is caused by the basidiomycete fungi Ustilago maydis. The disease is easily identifiable by its characteristic symptoms, which include the formation of galls on various aboveground parts of the maize plant, particularly in the ear. Although common smut has been documented in most maize growing regions, economic losses due to U. maydis are rare. In Central America, the galls formed by Ustilago maydis are consumed as a delicacy known as huitlacoche.

Pathogen: Ustilago maydis (syn. Ustilago zeae).

Symptoms

Symptoms of common smut are easily identified by the characteristic galls that form on most aboveground parts of the maize plant, including ears, stalks, tassels and leaves. Galls are initially greenish/grey-white with a shiny periderm. As galls mature they change to a dark grey color and contain masses of dark grey, black spores (teliospores). As galls mature, the periderm dries and disintegrates, releasing teliospores.

Galls that form at the ear tip are most distinctive and can reach 15cm in diameter. Galls often form on the ear tip. Galls on the leaf typically form on the midrib and distort the leaf, but rarely exceed 1.5cm in diameter. Young plants are most vulnerable as infection of the meristemic tissues can result in stunting or plant death.

Common smut is easily distinguished from head smut by the lack of vascular bundles in smut-infected years. Additionally, the gall periderm formed by common smut is thicker and remains intact longer in comparison with head smut.

Confirmation

Common smut is usually easily identified by its characteristic symptoms. It is easily distinguished from head smut by the thicker periderm layer, formation of numerous galls in the ear that replace kernels rather than the entire ear, and the absence of vascular bundles within galls that form in the ear.

Microscopic examination of teliospores produced within galls can also be used to confirm identity. Teliospores are generally spherical, 8-11µm in diameter, dark brown to black, and spiny. Teliospores are diploid and germinate to give rise to four or more fusiform, haploid sporidia. Occasionally, however, sporidia can be diploid. Plants can be infected by hyphae that emerge directly from the teliospore as well as diploid sporidia, which arise from the fusion of compatible haploid sporidia.

Incidence and factors favoring disease severity

Common smut has been reported from most maize growing regions globally, but is more severe in humid, temperate environments than in hot, humid, tropical lowlands. Nitrogen fertilizer increases disease severity. Conditions that lead to poor pollination, such as extended dry periods during silking, may favor common smut as silks remain susceptible to infection for longer periods.

Factors that lead to plant injury such as strong winds, hail, insect damage and blowing soil favor disease severity as the U. maydis predominantly infects plants through wounds. Teliospores are disseminated by wind and by the movement of infected, unshelled maize ears.

Host range

U. maydis is known to infect maize and Teosinte.

Geographic distribution

Life cycle

Teliospores are released from ruptured galls and overwinter in the soil, where they remain viable for several years. Under favorable environmental conditions, teliospores germinate to form haploid sporidia. Sporidia of compatible mating groups then fuse to form diploid mycelia that can infect maize plants. Teliospores can also directly infect maize plants. Infection usually occurs through wounds and injury on the maize plant or through the silks. Host cells around the infective hyphae expand, leading to the production of galls. Ears are infected through the silks and kernels are replaced by the production of galls. Infection of ears is uncommon following pollination. Healthy kernels from infected ears can be coated in teliospores but common smut is not recorded to be seed borne.

Damage

• Mechanism of damage:Damage is caused predominantly by the replacement of kernels with galls. Infection of meristemic tissue in young plants can also lead to plant death, while formation of galls on the stems can interfere with translocation of water and nutrients leading to yield loss.
• When damage is important: Damage is most critical when young plants are infected or when formation of galls on the stalks interferes with translocation of nutrients and water. In some cases severe infestation can lead to most kernels in the ear being replaced by galls.
• Economic importance:Losses due to U. maydis are rare as resistant varieties are widely available. Nonetheless, economic loss as high as 20% has been attributed to the disease when susceptible varieties have been grown under conditions favoring disease severity.

Global distribution

Common smut is found in most maize growing regions worldwide. In Central America, galls are consumed as a delicacy known as huitlacoche and susceptible varieties are cultivated.

Management principles

Host resistance

• Maize varieties that are resistant to common smut are widely available and offer the most cost-effective and practical means of disease management.

Fungicide

• Efforts to control common smut through the application foliar fungicides and seed treatments have not been highly successful.

Cultural control

• Avoiding mechanical damage to plants will reduce plant injury, which is the primary means of infection by the fungi.
• Controlling insect damage (e.g. controlling maize borers) will also limit plant injury.
• Crop rotation is not effective, as teliospores remain viable for several years in the soil
• Removing galls before they rupture will limit the spread of teliospores but is not practical in large-scale maize cultivation.
• A well-balanced fertilizer regime will reduce disease severity. High levels of nitrogen fertilization increase disease severity, although application of phosphorous reduces disease incidence.

References

CAB International. 1986. Ustilago zeae. Distribution Maps of Plant Diseases. Edition 6 (October), Map 93. Wallingford, UK: CAB International.

Draper, M.A. 2004. Common corn smut. Plant disease management in South Dakota, Cooperative Extension Service. Factsheet FS918.  http://agbiopubs.sdstate.edu/articles/FS918.pdf (28 August 2007).

Miller, S.A., P.E. Lipps and R.C. Randall. Common Smut of Corn, The Ohio State University Service. Extension Factsheet HYG-3119-96. http://ohioline.osu.edu/hyg-fact/3000/pdf/3119.pdf (28 August 2007).

Pataky, J.K. 1999. Head Smut. In Donald G. White (ed), Compendium of Corn Diseases. St. Paul, Minnesota: The American Phytopathology Society. Pp. 33.

Pataky, J.K. and M.A. Chandler. 2003. Production of huitlacoche, Ustilago maydis: timing inoculation and controlling pollination. Mycologia 95: 1261-70.

Contributor: Biswanath Das