Diseases:
Common bunt, stinking smut or covered smut
Dwarf bunt

Pathogen:

Common bunt of stinking bunt is caused by two fungi: Tilletia tritici (Bjerk.) Wint. (syn. T. caries) and T. laevis Kühn. (syn. T. foetida)

Dwarf bunt is caused by T. controversa Kühn

Symptoms:

• Obvious symptoms of these diseases are not apparent until stem elongation starts.
• The diseases caused by Tilletia tritici and T. laevis are identical but the fungi differ in spore morphology.
• Plants with dwarf bunt attain only one-quarter to one-half the height of a normal height of healthy plants, hence the name of dwarf bunt.
• Plants infected with common bunt show only a slight reduction in height.
• Main symptoms caused by common and dwarf bunt are the production of fungal structures called “bunt balls” in the spikes of wheat which replace healthy kernels.
• Bunt balls resemble kernels however they contain masses of black powdery fungal spores called teliospores.
• During harvest or upon crushing, the bunt balls rupture and release spores resulting in the contamination of wheat grain.
• Ears of either stinking bunt or dwarf bunt remain green longer than healthy plants, and the glumes and awns spread apart exposing the bunt balls. The latter symptom may be more pronounced with dwarf bunt.
• These diseases produce a pungent, fishy odour caused by the spores.
• Infected spikes are bluish green in colour and tend to maintain their green colour longer than healthy spikes.
• The bunt balls produced by Tilletia tritici and T.laevis are about the same size as the kernels they replace. Whereas, those produced by T. controversa are near spherical.

Confirmation:

• As common bunt and dwarf bunt have many commonalities in the symptoms they cause in wheat, microscopic observation of the spores is required for characterization of the bunt species.
• Tilletia tritici has a reticulate pattern on teliospores, whereas teliospores of T.laevis are smooth.

Why and where it occurs:

• The geographical distribution of dwarf bunt is limited to areas where winter cereals are grown with prolonged snow cover (Europe, USA, Canada, Near east). In the southern hemisphere it is also found in Argentina and Uruguay.
• Dwarf bunt tends to be localized to altitudes of 300-1000 m; years with frequent snow falls are usually associated with serious attacks.
• Dwarf bunt and common bunt can infect wheat plants either through the seed (seed-borne) or from the soil (soilborne).
• Common bunt has a worldwide distribution and is primarily a seed-borne disease although the fungus can survive in the soils during the dry summers.
• Common bunt spores germinate over a wide temperature range 5-20°C. Dispersal of common bunt is mainly by movement of grain and farm machinery contaminated with spores.
• Dwarf bunt is harder to control since spores of the fungus can survive over 10 years in the soil.
• Dwarf bunt spores germinate only at low temperatures (optimum 0-8°C) over a period of several weeks. This normally happens under snow cover on unfrozen ground during the winter months.

Causal agent or factors:

• T. tritici and T. laevis which cause common bunt are widely distributed; T. tritici is the more common of the two fungi. Both fungi may occur at the same time in the same infected plant.
• Both T. tritici and T. laevis produce thick walled globose teliospores.
• Both T. tritici and T. laevis contain trimethylamine a volatile chemical that has a characteristic fishy odour.
• T. controversa is closely related to T. tritici and was once considered a strain of the latter fungus.
• Both T. tritici and T. controversa teliospores have reticulated walls, however those of T. controversa are broader and deeper with a gelatinous sheath.
• Spores of T. tritici are slightly smaller than those of T. controversa (14-23.5 µm versus 19-24 µm).
• Bunted plants are weaker than healthy plants, and are often susceptible to seedling blights.
• Dwarf bunt is difficult to control because of the resistant resting spores which remain viable in the soil for a number of years and because most seed-applied fungicides are not effective.

Host range:
Triticum aestivum (bread wheat), Triticum turgidum (durum wheat), Triticale, Secale cereale (rye), Hordeum vulgare (barleys) and wild grasses.

Damage:

Mechanism of damage

• Teliospores of common bunt that persist in the soil or found on contaminated seeds, germinate at the same time as the wheat seeds germinate. The fungus invades the coleoptiles of the developing seedlings prior to emergence. The fungus then invades deeper into the young tissues and establishes itself in the tissues that eventually develop into the spike. In susceptible cultivar the fungus inhabits the developing kernels as the plant grows and displaces the tissues within the kernels. When the field is combined during harvest, the disease cycle is repeated due to the release of teliospores from mature bunt balls. These teliospores are dispersed by wind and contaminate seeds and soils, the process is important in the distribution of new strains of bunt fungi.
• The primary source of inoculum of dwarf bunt is through teliospores in the soil. Teliospores germinate at low temperatures, under the snow or near the soil surface. They invade susceptible wheat cultivars at the two to three leaf stage.
• Smut and bunts are typical examples of monocyclic plant diseases. Losses are linearly dependant on infected plants.

When damage is important:

• Dwarf bunt is a serious disease, particularly of winter wheat at relatively high altitudes. It is very difficult to control because of the resistant resting spores which remain viable in the soil for a number of years and because most seed-applied fungicides are not effective.
• Common bunt is potentially very important in most wheat-growing areas of the world. The most common source of infection is via sowing of contaminated seed from the previous crop.

Economic importance:

• Common bunt has a greater effect on the value of wheat because contaminated grain has an objectionable odor and cannot be used for human consumption.
• Even a low incidence of of smut can result in the grain being downgraded.
• Untreated, common bunt can destroy more than 50% of grain, but on average the losses are usually 5-10%.
• Dwarf bunt only affects a small portion of the total wheat production worldwide. However due to the presence and persistence of the disease, its economic importance is heightened due to the possibility of spread of the disease to uninfected areas.
• Some countries have quarantines regulations prohibiting the import of grain containing teliospores of T. controversa. The high similarity with teliopspores of T.tritici may exacerbate the problem.

Management principles:

• The most effective control of common bunt is through the use of seed-treatment fungicides.
• Common bunt can be controlled by seed treatment with carboxin, some benzimidazoles and difenoconazole.
• A seed dressing with difenoconazole provides nearly complete control of dwarf bunt but carboxin is ineffective.
• An additional practice that may help control common bunt is to plant wheat early when soils are warm (greater than 25 °C) because infection by the fungi is favored in cool soils.
• Application of a systemic fungicide may be more effective in the control of dwarf bunt as the fungus infects wheat seedlings after they are established.
• Strict international quarantine limits dissemination of dwarf bunt disease through contaminated wheat seeds.
• Where possible the use of wheat cultivars resistant to common and dwarf bunt should be used.
• Resistant cultivars may be used to control both smut diseases but races are existing and the pathogens are highly variable
• Race-specific resistance is effective but must be combined with seed treatment to control stinking bunt in areas where soil-borne inoculum is important.

References:

Baute, T. (ed.) 2002. Cereals: Dwarf Bunt and Common Bunt (Wheat). In Agronomy Guide for Field Crops. Guelph: Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). Available online at http://www.omafra.gov.on.ca/english/crops/pub811/6bunt.htm

Fuentes-Dávila, G., B.J. Goates, P. Thomas, J. Nielsen and B. Ballantyne. 2002. Smut diseases. In B.C. Curtis, S. Rajaram and H. Gómez Macpherson (eds.) Bread Wheat: Improvement and Production. Plant Production and Protection Series No. 30. Rome: FAO of United Nations.

Jardine, D.J. Common bunt or stink smut of wheat. Fact sheet (AC-7-96) Kansas Extension plant pathology, Kansas State University.

Mamluk, O.F. 1998. Bunts and smuts of wheat in North Africa and the Near East Euphytica 100:45-50.

Murray, T.D., D.W. Parry and N.D. Cattlin. 1998. A Color Handbook of Diseases of Small Grain Cereal Crops. Ames, IA: Iowa State University Press.

Wiese, M.V. 1987. Diseases caused by nematodes. In M.V. Wiese (ed.), Compendium of wheat diseases. St. Paul, MN: The American Phytopathological Society (APS Press).

Wilcoxon, R.D. and E.E. Saari (eds.). 1996. Bunt and smut diseases of wheat: concepts and methods of disease management. Mexico, D.F.: CIMMYT.

Contributors: H. K. Buhariwalla, E. Duveiller, and P. Kosina