Karnal Bunt Disease
Research forcuses on its persistence in soil

How serious is Karnal bunt and what should Arizona growers do about it? Does this controversial fungus disease of wheat and other small grains, identified on four kernels of Arizona durum seed wheat in 1996, merit a zero tolerance policy in certified grain? And is it possible to completely eradicate it from a field?

Growers, regulatory agencies and scientists have different answers to these questions, but they all agree that more information is needed about how the Karnal bunt fungus survives. When the bunted kernels were found in Arizona in 1996, a federal quarantine was placed on Arizona wheat, restricting its movement. Because the United States is the world's largest exporter of wheat, the policy regarding this disease has far-reaching implications for growers. Twenty-one foreign countries have rejected wheat containing Karnal bunt.

In spite of these extreme measures, members of the American Phytopathological Society, including Merritt Nelson, head of the Department of Plant Pathology at The University of Arizona, maintain that it has always been regarded as a minor disease of wheat. They say it pales in comparison to diseases of wheat such as stem rust, which also is not a problem in Arizona, but which cuts yields significantly in other wheat growing areas.

The current argument centers more on the political aspects of trade agreements and what other markets will accept, but the role of scientists in the UA College of Agriculture, as members of the Karnal Bunt Task Force, has been to objectively examine in greater detail the way the fungus spreads and operates.

"We believe it is important for us to be involved in this research, because of its impact on growers," Nelson says. It's a way to help agencies and growers make decisions about planting, soil management, and postharvest crop handling. It all begins with the way the fungus travels.

"This disease is spread by the wind," Nelson says. "The scientific literature shows that burning trash from an infected wheat crop ejects spores into the stratosphere, up to 3,000 meters (10,000 feet) where they can travel for miles. Suppressing it is possible; eradicating it is not." Nelson believes the spores of the fungus are already widely distributed over the United States and possibly Canada. Once spores are in the soil, the disease will only develop when the climate is just right, and even then, there must be a very large number of spores present.

"It takes a tremendous amount of inoculum in the field to see even a small amount of the disease," Nelson says. He believes the fungus has actually been present in the U.S. for several years, but was not detected until more sophisticated methods of diagnosing it were developed.

In the life cycle of the fungus, the stage called the teliospore (thick-walled resting spore) develops in the infected kernels, then drops into the soil where it has been reported to live for up to five years. Not enough is known, however, about how soil conditions and farming practices affect the persistence of the teliospore in the soil.

"I am going to be working with scientists at APHIS/USDA on a project to determine the survival capability of these teliospores in Arizona soils," says Mary Olsen, a plant pathologist in the UA College of Agriculture. "We'd like to compare irrigated and non-irrigated soil to find how long the teliospores remain alive (viable) and to check the survival of the spores at different depths."

To do this, Olson will bury samples of soil infested with Karnal bunt teliospores at different depths in field plots at the UA Campus Agricultural Center in Tucson. The soil has been analyzed and sent to the APHIS research facility in Maryland where it will be infested. The soil will be put into special bags that prevent the spores from escaping and the plots will be fenced.

"We'll bury them in the soil and dig up a few every three months over a period of three years," Olson says. "We'll send samples back to APHIS as we take them out, so scientists there can determine how many spores have survived."

The results of these studies will assist growers in choosing cultivation and rotation schedules that discourage the growth and spread of the fungus. Future studies will target the effect of cultivation, plants and competition from other microorganisms in the soil on survival of the fungus.

What is Karnal Bunt?

Named for the city of Karnal, India, where this fungus disease first appeared on a wheat crop in 1931, Karnal bunt damages seed heads by replacing kernels with a brown-black, powdery mass of foul-smelling spores. Flour made from a crop that is infected at the rate of 3% or more will have a fishy odor and taste, making it unfit for human consumption from a cosmetic standpoint. While the fungus reduces the quality of the crop, no serious yield reductions have been reported in India or more recently, in Mexico, where the fungus appeared in 1970. The fungus is nontoxic to humans and livestock.

Caused by the fungus Tilletia indica, the disease is also known as partial bunt because it usually damages only part of a kernel. The strict environmental conditions that favor the growth of the fungus involve the precise timing of high humidity and/or rain occurring during the flowering of the crop, at the same time the spores are blowing across a field. Crops affected include common wheat, durum wheat, rye and triticale.

The disease is difficult to diagnose in the field because the pathogen grows inside the seedhead, and because it usually does not infect a large number of plants or even an entire plant. Seed samples must be washed and centrifuged in a laboratory to find the spores of the fungus. Chemical seed treatments, fungicides and fumigation provide adequate control.

Article written by Susan McGinley, ECAT, College of Agriculture
This is part of the 1997 Arizona Experiment Station Research Report
This document is located at http://ag.arizona.edu/pubs/general/resrpt1997/karnal.html
Return to Index for 1997 report
Researchers:

Merritt Nelson Department of Plant Pathology
(520) 621-1828
mrnelson@ag.arizona.edu

Mary Olsen
Department of Plant Pathology
(520) 621-1828
molsen@ag.arizona.edu