Karnal Bunt of Wheat

Karnal Bunt of Wheat

The spread of Karnal bunt has been closely monitored since it was discovered near the city of Karnal in Punjab, India in 1931. The disease reached the Western Hemisphere in 1972 when it was discovered in the Mexican state of Sonora. The first report of Karnal bunt in the United States came from Arizona in March of 1996. A significant reduction in yield and quality is seldom associated with this disease. However, due to its mode of dissemination and control difficulties many countries have imposed strict quarantines on the importation of infected seed. In many cases, any seed produced from an area where the disease has been identified cannot be imported. The potential for economic losses as a result of import restrictions has led to an extensive Karnal bunt survey in the U.S. The causal organism is Tilletia indica Mitra = Neovossia indica (Mitra) Mundk. Common or bread wheat is the host most affected by the pathogen but durham wheat and triticale are also susceptible. There is a great deal of genetic variability within the species but researchers do not agree on the designation of specific races. The fungus survives between crops as teliospores. The spores are soil or seed-borne and can survive up to 5 years on either medium. Spores are disseminated on or in infected seed as well as on transportation vehicles and harvesting and threshing equipment. They are also windblown. Teliospores require a dormant period before they will germinate. Poor germination may last up to 9 months. After the dormancy period spores on the soil surface germinate at an optimum temperature range of 15^o to 25^oC. Moisture is the most critical factor in germination requiring a relative humidity of 82% or higher, preferable with the presence of free water. A basidium (promycelium) emerges from the teliospore and basidiospores (primary sporidia) are produced on this structure. Primary sporidia germinate on the soil or are wind blown to lower leaf leaves and germinate to form secondary sporidia which are forcibly ejected. Secondary sporidia can germinate and form more spores or invade host tissue. They are also wind disseminated. Infection of host plants occurs during a two- to three week period near or at anthesis. Secondary sporidia invade glumes through stomata. The fungus grows intercellularly down to the base of the glume and into developing kernels. It may or may not invade the embryo and mycelial growth is restricted by the pericarp. Infection commonly occurs in a few kernels per head and a few heads per plant. Mycelium splits and develops into teliospores in much the same way as common bunt. The mass of spores that are produced is referred to as a sorus. The sorus commonly replaces part but not the entire kernel. Teliospores remain encased in the pericarp until it is ruptures during harvest and they are released.

Symptoms

Symptoms of Karnal bunt are often difficult to distinguish in the field due to the fact that incidence of infected kernels on a given head is low. There may be some spreading of the glumes due to sorus production but it is not as extensive as that observed with common bunt. Symptoms are most readily detected on seed after harvest. The black sorus, containing dusty spores is evident on part or all of the seed, commonly occurring along the crease. Heavily infected seed is fragile and the pericarp ruptures easily. The foul, fishy odor associated with common bunt is also found with Karnal bunt. The odor is caused by the production of trimethylamine by the fungus. Seed that is not extensively infected may germinate and produce healthy plants.

Plant Health Management

Quarantine.

Karnal bunt is difficult to control because it is seed and soil borne and can survive for long periods of time outside its host. The focus of quarantines has been to restrict movement of the pathogen from country to country and within countries where it has been detected.

Eradication.

Once Karnal bunt is identified in a given area every attempt is made to eliminate it by destroying infected plants and seed. Rotation to durham wheats and row crops has also been used as a means of eliminating the pathogen from a given area.

Resistant Cultivars.

Resistant cultivars have been identified in other countries. However, a high degree of resistance is not currently available in hard red winter wheats adaptable too much of the Great Plains.

Fungicide Application.

Foliar applied and seed treatment fungicides have been assessed for their ability to control the disease. Compounds are not currently labeled for use on this disease in the U.S. other than for experimental research or emergencies.

References

Bonde, M.R., peterson, G.L., Schaad, N.W. and Smilanick, J.L. 1997. Karnal bunt of wheat. Plant Dis. 81:1370-1377.

Bowden, B., McCluskey, P. and Sim. T. 1996. Karnal bunt, questions and answers. Cooperative Extension Service, Kansas State University, Manhattan, KS. MF-2220.

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

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

A flurry of activity has surrounded the detection of Karnal bunt in the United States. Surveying for and containing the causal organism have been costly but necessary to maintain our world market for wheat exportation. The article by Bonde, et.al. gives an excellent account of the short but turbulent history of this disease in the U.S. as well as predictions for its future

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