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Ustilago maydis (maize smut)

Maize smut is an economically important fungus which infects the stems, leaves and flowers of sweetcorn and may cause severe crop losses.

 Ustilago maydis on sweetcorn

Ustilago maydis on sweetcorn (Photo: Geoffrey Kibby)

Species information

Common name: 

corn/maize smut, boil/blister smut of corn

Conservation status: 

The smut is generally common throughout its range and is also an economically important pathogen, sometimes causing severe crop losses. It is not considered of conservation concern.

Habitat: 

Parasitic on maize (Zea mays) and related species, causing markedly swollen tissue in the flowering parts, stems and leaves.

Key Uses: 

Food, herbal medicine.

Known hazards: 

See below.

Taxonomy

Kingdom: 
Fungi
Phylum: 
Basidiomycota
Order: 
Ustilaginales
Family: 
Ustilaginaceae
Genus: Ustilago

About this species

Ustilago maydis (previously known as U. zeae) is native to Mexico, but is now distributed almost worldwide wherever the host is cultivated. It was first described in 1815 by de Candolle from France, as Uredo maydis. The economic importance of this species has led to an extensive literature on its biology, taxonomy, and uses. It is an important crop pathogen and a major quarantine pest. Host infection occurs through young tissue above ground, often via wounds, affecting all plant parts and often leading to large, distinctive and conspicuous galls of the host.

Control measures mainly involve the use of resistant host varieties and chemical seed treatments. However, the swollen host tissue caused by this smut has long been an important food item, known as huitlacoche in Central America, and is becoming increasingly so in North America. Corn smut is also an important study organism which has been extensively used in genetic research and subjected to detailed study.

Genus: 
Ustilago

Discover more

Geography and distribution

Widespread. Native to Central America and now almost worldwide in distribution though rarer in tropical regions, and absent from New Zealand. Since 1982 it has become re-established in Australia where, after being first recorded in New South Wales in 1911, it was eradicated in about 1940. Quite common in Europe wherever maize is grown.

Description

The presence of Ustilago maydis can be recognised by large irregular swellings, of up to 20 cm across, on the stems, leaves and flowering parts of maize plants.

These swellings (galls) are silvery-white at first, and covered by a thin greyish membrane, which soon darkens, and ruptures at maturity to expose the dark brown, powdery spore mass. The spores are rounded and pale brown, with a dense covering of small pointed spines.

History of maize smut

Maize smut is the oldest plant disease which is illustrated by drawings, infected maize plants being figured in the Florentine Codex MS prepared soon after the Spanish conquest of Mexico in 1519. The galls were certainly well known to the Aztecs long before the 16th century, and feature in early Mexican mural paintings.

The smut was first recorded in Europe in 1768 and then referred to as Lycoperdon zeae, the generic name now applied to puff-balls. In Britain it was first recorded in Dorset in 1850, an observation published by Rev. M.J. Berkeley in the Gardener’s Chronicle.

Uses

Used traditionally as human food, especially as soup, maize smut is much prized as a delicacy, and has long been marketed as a food in Mexico. It is of increasing economic importance in North America, and grown more extensively as a cash crop which may be more valuable than the maize itself. The ethnic name for the young galls is huitlacoche. The flesh contains carbohydrates, fats and proteins, and has high levels of essential amino acids, oleic and linoleic acids, and vitamins.

The smut has also had other uses by native Americans, such as inducing labour, due to ustilagine which is similar in its properties to ergotamine obtained from ergots (Claviceps purpurea). It is also valued as a homeopathic remedy, being used, in accordance with the medical descriptions, primarily for diseases of the female reproductive organs.

Known hazards

The swollen host tissue is a popular and economically important food in Mexico and elsewhere, regarded as a delicacy and considered to be highly nutritious. However, there are reports of toxicity and effects such as dermatomycosis (fungal infection of the skin) and hair loss in humans, other allergic disorders, and pathological effects in animals. These seem to have little foundation, although Ustilago maydis is known to produce various secondary metabolites including ustizein, guanacine, itaconic and ustilagic acid, and the alkaloid ustilagine, which may have similar effects to ergotamine, obtained from ergots (Claviceps purpurea).

Maize smut at Kew

Ustilago maydis is occasionally recorded galling the stems and flowering parts of maize in the grass garden, but is not often seen.

Preserved specimens of Ustilago maydis from throughout its range are maintained in the Kew Mycology Herbarium and, although not accessible to the general public, are available for study by workers worldwide.

References and credits

Boericke, W. (1987). Homeopathic Materia Medica with Repertory. Homeopathic book service, London.

Checklist of the British and Irish Basidiomycota. Available online.

Fischer, G.W. & Holton, C.S. (1957). Biology and Control of the Smut Fungi. New York: Ronald Press.

Fox, R.T.V. (1990). Fungal Foes in your Garden 9. Maize Smut. Mycologist 4: 142.

Fungus Records Database of Britain and Ireland. Available online.

Holliday, P. Fungus Diseases of Tropical Crops. Cambridge University Press.

McMeekin, D. (1999). Different perceptions of the Corn Smut fungus. Mycologist 13: 180 – 183.

Mordue, J.E.M. & Ainsworth, G.C. (1984). Ustilaginales of the British Isles. Kew: CMI.

Preece, T.F. (1993). Maize cob galls caused by the smut fungus Ustilago zeae. Cecidology 8(2): 88 - 90.

Valverde, M. E., Paredes-Lopez, O., Pataky, J. K. & Guevara-Lara, F. (1995). Huitlacoche (Ustilago maydis) as a food source ― biology, composition, and production. Critical reviews in food science and nutrition 35: 191 - 229.

Vánky, K. & Shivas, R.G. (2008). Fungi of Australia. The Smut Fungi. ABRS, Canberra; CSIRO Publishing, Melbourne

Kew primary author: Brian Spooner

While every effort has been taken to ensure that the information contained in these pages is reliable and complete, the notes on hazards, edibility and suchlike included here are recorded information and do not constitute recommendations. No responsibility will be taken for readers’ own actions.

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