APSnet: Intro to Karnal bunt
June 24 - August 18, 1996

Karnal Bunt Symposium

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In early March, 1996, Karnal bunt was found in 1995 certified seed lots of irrigated durum wheat planted in the Gila Bend area of southwestern Arizona, south central New Mexico and in the vicinity of El Paso, Texas. Emergency quarantines have been placed on infected properties, seed, farm equipment, planted wheat, and soil in the areas planted with seed infected by Karnal bunt.

Background Information

Karnal bunt, also called partial bunt, was first reported in 1931 near the city of Karnal, Punjab, India. Caused by the fungus, Tilletia indica, the disease is widespread in northwest India and in adjacent areas of Pakistan, Iraq and Afghanistan. More recently it has been introduced into Mexico (1970), and has been reported in northern Mexico since 1982. This is the first report of Karnal bunt in the United States. Karnal bunt affects common wheat (bread wheat) and to a lesser degree durum wheat, triticale, and related species. The disease significantly reduces seed quality, but only minimal effects on yield have been reported. Karnal bunt infected grain is not toxic to live stock or humans but can result in flour unfit for human consumption due to a strong fishy odor.

Developing wheat kernels are randomly and incompletely converted to smut sori. Normally, only a few seeds are attacked per head. Diseased heads are not conspicuous because the glumes are not noticeably distorted by infected kernels (as in common bunt, Tilletia tritici). Infected kernels are usually only partially eroded at their embryo end (Graphic: plate 13, from APS Wheat Compendium). Larger sori may extend along the crease and occasionally envelop the whole kernel. The delicate pericarp surrounding each sorus of bunt spores (teliospores) is easily broken during harvest. The brown-black spores have a fishy odor similar to those of common bunt.

The Fungus

Karnal bunt is caused by the fungus, Tilletia indica Mitra (also known as Neovossia indica) which produces large, globose, dark-brown spores called teliospores ranging from 22-49 microns in diameter. The teliospores bear truncate projections surrounded by a delicate membranous sheath. Upon germination, stout structures called promycelia emerge and numerous (frequently over 100) haploid (N) spores called sporidia form at their apex. These primary sporidia do not fuse as the spores of other types of bunt fungi do but germinate directly to form thread-like fungal growths called hyphae OR they may germinate to form uninucleate secondary sporidia, which are forcibly discharged.

The Disease Cycle

Bunt teliospores persist in soil for up to five years and on seed. Soilborne teliospores are the principal source of primary inoculum. Teliospores germinate at or near the soil surface in response to free moisture and produce primary and secondary spores called sporidia. The sporidia are wind-dispersed and appear to infect spikelets by directly penetrating the glumes and ovary wall. Promoted by cool, humid or wet weather, infections lead to partial and occasionally, complete conversion of developing kernels into darkened masses of teliospores. At harvest, the pericarp of bunted kernels is easily broken, liberating teliospores that contaminate soil and seed. Partially bunted seeds often retain their capacity to germinate and produce healthy plants.

Disease Management

Where the required conditions of temperature and moisture occur regularly, Karnal bunt is difficult to control. Fungicides used to control other bunt and smut diseases are less effective against Karnal bunt since Karnal bunt infects wheat plants during flowering. Chemical seed treatments inhibit the germination of teliospores on the seed but not those inside. Some fungicides applied at heading give partial protection against infection but have been ineffective in controlling the disease. Fumigation of seedlots has also been ineffective. Wheat cultivars differ in susceptibility to Karnal bunt, but none are known to be immune. Durum wheats and triticales are less susceptible than bread wheats. Quarantines are imposed by many countries to prevent the introduction or spread of Karnal bunt.

Current Situation

On March 25, 1996, the Animal and Plant Health Inspection Service (APHIS) issued a federal quarantine on wheat and triticale following the identification of Karnal bunt spores on durum seed. The quarantine originally involved the state of Arizona and six adjacent counties in Texas and New Mexico. Several counties in California were added later. This measure was taken to restrict movement of the fungus into non-infected areas of the United States.

Potential Impact on US

The presence of Karnal bunt in the United States will have a significant impact on the U. S. farm economy. The United States is the world's largest wheat exporter, providing one-third of the world wheat exports, valued at approximately $5 billion in fiscal year 1995. International quarantine restrictions prevent importation of wheat from a country in which Karnal bunt has been confirmed, regardless of the size or isolation of the area affected. Currently 21 countries have Karnal bunt on their quarantine list but several have relaxed their regulations to allow restricted movement of wheat into their countries.