Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2003
Publication Date: 8/1/2003
Citation: Marshall, D.S., Work, T.T., Cavey, J.F. 2003. Invasion pathways of karnal bunt of wheat into the united states. Plant Disease. Interpretive Summary: The wheat disease, Karnal bunt is named after the location where it was first found, Karnal, India. A fungus that is spread by infected wheat grains causes the disease. The disease is a minor problem in wheat production, but is a major problem for wheat exports because the countries that import wheat have a "zero tolerance" policy for the disease. In the United States, Karnal bunt was first found in Arizona in 1996. It was subsequently found in harvested wheat grain in Texas in 1997. The disease then disappeared until it was found again in Texas in 2000. We examined the potential pathways of arrival of Karnal bunt into the United States by analyzing when, where, and how USDA/APHIS inspectors between 1984 and 2000 intercepted the disease. We found that the disease was intercepted only along land borders with Mexico and on parcels originating in Mexico that were intercepted at U.S. airports. Because Karnal bunt had ample opportunity to enter the U.S. in each year from 1984 to 2000, we surmised that the fungus might need a long time to build-up its numbers after arrival in an area before becoming a noticeable problem.
Technical Abstract: Karnal bunt of wheat (caused by Tilletia indica Mitra) was first detected in the United States in Arizona in 1996. The seed lots of infected, spring-habit, durum wheat associated with the initial detection were traced to planted fields in California, Arizona, New Mexico, and Texas. However, in the summer of 1997, the disease appeared in unrelated, winter-habit, bread wheat, located over 700km from the nearest potentially contaminated wheat from 1996 (and destroyed prior to reinfection). Here, we examined potential invasion pathways of the fungus associated with the movement of wheat into the United States. We analyzed the USDA/APHIS Port Interception Network (PIN) database from 1984 through 2000 to determine likely pathways of introduction based on where, when, and how the disease was intercepted coming into the United States. All interceptions were made on wheat transported from Mexico, with the majority (98.8%) being intercepted at land border-crossings. Karnal bunt was not intercepted from any other country over the 17 year period analyzed. Most interceptions were on wheat found in automobiles, trucks, and railway cars. The majority of interceptions were made at Laredo, Brownsville, Eagle Pass, and El Paso, TX; and Nogales, AZ. Karnal bunt was intercepted in all 17 years, however interceptions peaked in 1986 and 1987. In most years, interceptions were made in the month of May, however interceptions could be found during all months of the year. Our results indicate that Karnal bunt has probably arrived in the United States on many occasions, at least since 1984. We surmised that because of the relatively unaggressive nature of the disease, and its reliance on rather exacting weather conditions for infection, that it is possible this disease has a long period of latent survival between initial arrival and becoming a thriving, established disease.