Karnal Bunt in Mexico

Dr. Fuentes is a specialist in smuts and bunts and has worked on Karnal bunt (KB) for the last 8 years. He is a recognized authority on KB.

In Mexico, Karnal bunt was first recorded in 1969-70 in the Yaqui and Mayo valleys in the state of Sonora. There is no record of phytosanitary measures being taken to control dissemination of the pathogen between that time and 1983. Seed production and exports were economically important activities particularly in the Yaqui valley from 1964 to 1981, where thousands of tons were shipped to the USA, China, India, Pakistan, Brazil, Argelia, Libya, Syria, Greece, Spain, etc. During that period most bread wheat commercial cultivars were susceptible to the disease (although this was not known at that time). Disease incidence levels were noticeable in 1981-82 and in 1983, the Sonora Department of Agriculture implemented regulatory measures in an attempt to control the disease and to prevent dissemination of the pathogen to other wheat-producing areas. One of the most important regulatory measures was the restriction on bread wheat planting in fields where disease incidence was over 2% infected kernels/kg. However, this measure has not only caused important economic losses to farmers, but it does not help to minimize the disease since the area is highly infested. In 1987, the federal government issued internal quarantine No. 16 against Karnal bunt (partial bunt) in which other important measures included: regulation and tolerance levels of infected seed for seed production, fumigation of commercial grain with methyl bromide, seed treatments, and restrictions on movement of agricultural machinery. More recently, Mexico's Department of Agriculture has recommended the cultivation of tolerant bread wheat cultivars Arivechi for south Sonora and Guamuchil for northern Sinaloa. These cultivars were released for commercial use by the Mexican Institute for Agricultural Research and CIMMYT during 1992.

Despite the massive wheat seed distribution to many wheat-producing areas in Mexico and around the world from the Yaqui valley in the 60', 70's and early 80's, and without phytosanitary measures in effect, the disease has been confined and established to areas of northwestern Mexico reaching approximately 400 km north, south and west from the original inoculum source. Disease incidence has varied from one year to another, since it is highly affected by weather conditions. Noticeable presence of the disease occurs when high relative humidity-rain coincide with heading-flowering of the wheat plant. Years when disease incidence has been extremely low in the Yaqui valley are 1982, 1984, 1987, 1988, 1990, 1994 and 1996; wheat surveys indicate that 85 to 99.7% of the samples analyzed were free of the disease. Years with the highest numbers of grain samples with Karnal bunt are 1983 and 1985. However, even in those years, the number of samples heavily infected (with more than 1% infected kernels) were 7.5 and 11.2%, respectively. It is known that farmers whose fields contain levels higher than 3% infected kernels, make mixtures with healthier lots in order to be able to sell their product to the industry. Despite this, there are some farmers in some years, who are economically affected by the disease when they have to destine their wheat for animal feed.

It has been estimated that yield loss accounts for only 0.12% per year in the affected areas of northwest Mexico, and very importantly, the fungus does not produce toxins. However, the high level of importance given to Karnal bunt has been brought up by the regulatory agencies, which in turn has been translated into an issue of commercial and political significance.