Common Bunt of Wheat

Common Bunt of Wheat

Common bunt has been recognized as an infectious disease since the 1700's. The disease is also referred to as stinking smut or covered smut. Despite advances in control measures the disease continues to be a threat to wheat producers, especially in areas where seed treatment is not practiced. Bunt can reduce the quantity and quality grain. It also poses a health and safety hazard to field workers. The spores are an allergen to many people and large masses of spores easily become ignited causing explosions in harvesting and threshing equipment and storage units. Two fungi serve as causal organisms of the disease: Tilletia caries (DC.) Tul. & C. Tul. = T. tritici (Bjerk.) G. Wint. in Rabenh. and Tilletia laevis Kuhn in Rabenh. = T. foetida (Wallr.) Liro. Wheat is the primary host but triticale, barley, rye and several grasses may occasionally serve as hosts. The process of genetic recombination leads to a great deal of variability in virulence within these fungal species. A few races of the fungi have been identified but it is almost impossible to keep track of all the different races because they are constantly changing. This fact also makes breeding for plant resistance nearly impossible. The fungi overwinter as teliospores on seed and may also survive in the soil. Soil survival is most important when winter wheat is produced in an area with arid summers. Teliospores germinate and form a basidium (promycelium) in cool moist soil. The basidium gives rise to haploid (1n) basidiospores, also known as primary sporidia. These spores exist in different mating types that fuse via lateral conjugation hyphae. An H-shaped structure called a dikaryon (2n) is formed. Infectious hyphae or secondary sporidia are produced from the dikaryon. Secondary sporidia are forcibly ejected and germinate to form infectious hyphae or form more sporidia. Infectious hyphae invade coleoptile tissue directly through the cuticle before they emerge from the ground. Teliospore germination, production of infectious hyphae and host penetration occur at a soil temperature range of 40-60oF and soil moisture at 15 to 60% field holding capacity. The fungus invades the terminal meristem (growing point) and grows systemically in the host both inter- and intracellularly. Mycelia invade the developing ovary and replace the tissue that would normally form in the pericarp. The mycelium then splits off into dark, thick-walled fragments that develop into teliospores and a bunt ball is created. Each bunt ball may contain 1-4 million teliospores. These structures are easily ruptured during harvest and release spores that infest soil, seed, and harvesting equipment. Storage facilities also become contaminated when filled with infected seed. One cycle of bunt occurs per season. Fall seeded wheat may become infected from soil-borne teliospores if soil conditions have r emained dry during the summer and are cool at the time the seed is germinating. If viable teliospores are not present in the soil, seed borne spores are the primary source inoculum. Common bunt also occurs on spring seeded wheat but infection is less frequent and less damaging than it is on fall seeded crops. Infection with common bunt increases susceptibility to winter injury and other diseases including yellow stripe rust and seedling blights while it increases resistance to powdery mildew.

Symptoms

Plants infected with common bunt may go unnoticed until heading. Infected plants are shorter than normal plants because of shortened internodes. Infected heads are bluish green when they emerge from the boot rather than the normal yellowish green. Infected heads also maintain a greenish color longer than healthy heads. In addition, they are more slender than normal heads. Bunt balls, which are dull gray in color, are present in the head. It is common for the entire head to be infected but in some cases kernels may also be present. Bunt galls are approximately the size of a normal kernel but are more rounded and cause the glumes to spread apart. This feature gives the heads an open appearance and is easily visible on cultivars with awns. The balls are easily ruptured during harvest and a black cloud of spores may be visible in the wake of harvesting equipment. One of the most common signs of the disease is the fishy odor associated with the spores. The odor results from production of trimethylamine and gave rise to the "stinking smut" title for this disease. The presence of black spores on the seed and foul odor, which carries over into flour, reduce the market quality of infested wheat. Infested seed is also decreased in value as animal feed since livestock commonly refuse to eat it.

Plant Health Management

Seed Treatment.

This practice is one of the oldest and still the most effective way to control common bunt. The first "chemical" seed treatment began in the early 1800's with the use of copper sulfate. Compounds labeled for use today include carboxin (Vitavax), difenocanazole (Dividend), hexachlorobenzane (HCB), and pentachloronitrobenzene (PCNB) 1997.

Use cleaned/certified seed produced in bunt-free fields.

References

Agrios, G. N. 1978. Plant Pahtology, 2nd ed. Academic Press, New York. 703 pp.

Holton, C.S. and Tapke, V.F. 1953. The smuts of wheat, oats, barley. Pages 360-368 in Plant diseases, the yearbook of agriculture 1953. U. S. Dept. of Agriculture, Washington, D.C. House Document No. 122.

Mathur, S.B. and Cunfer, B.M. eds. 1993. Seed-borne diseases and seed health testing of wheat. Jordbrugsforlaget. Frederiksberg, Denmark. 168 pp.

Watkins, J.E. and Prentice, L.J. 1997. Diseases affecting grain and seed quality in wheat. Cooperative Extension Service, Institute of Agriculture and Natural Resources, University of Nebraska - Lincoln. EC97-1874.

Wiese, M.V. ed. 1987. Compendium of wheat diseases, 2nd ed. APS Press. St. Paul, MN. 112 pp.

Useful Links

Texas A&M University

Highlighted Area

The most obvious symptoms of common bunt are visible on the head. Most people are on the lookout for bunt galls and may miss the more subtle symptoms that occur on the flowers. Several changes take place in the appearance of floral structures when they become infected by these fungi. Pistils are longer while stamens are shorter than normal. Stamens also fail to extrude at flowering time. The anthers change in color from a healthy green to pale yellow. Infected ovaries, the site of teliospore production, are longer and wider than normal and are green instead of white.

Caveat

This description is presented for information only and no endorsement is intended for products listed, nor criticism meant for products not mentioned. Always consult the product label before purchasing and using any pesticide.

Material contained on the Links from the page are the responsibility of the linked page's author(s).

This page was researched and drafted by: Jane Christensen, Department of Plant Pathology, University of Nebraska-Lincoln

Disease images were provided by: Dr. David Wysong, Department of Plant Pathology, University of Nebraska-Lincoln

Questions, Comments, Complaints and Complements?

This page is authored and maintained by:
Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

E-Mail Home Page