Dr. Merritt Nelson, Senior Department Head, University of Arizona, Tucson, Arizona.
Karnal bunt (KB), a covered partial smut of wheat, caused by the fungus Tilletia indica, is relatively new in the wheat fields of the world. It is a minor disease when compared to other smuts and diseases of this crop but since it is not distributed uniformly in the wheat producing countries, there is sensitivity in world trade about buying wheat grown in countries that are known to have KB. Teliospores of KB were found in 1995 wheat samples from Central Arizona on March 8, 1996.
The disease was first identified in wheat fields near Karnal, India in 1931 and has since been found in other Asian countries and has been endemic in the Yaqui Valley in Mexico since 1969. It was not recognized as a significant disease in the Yaqui Valley until 1980 and was first detected in Hermosillo and Caborca in 1992. Since 1980, the disease has been very erratic. For example, 87 per cent of samples from Yaqui Valley wheat fields tested positive in 1984-85 but only 0.35 per cent positive in 1986-87. The disease is most common in coastal areas of Sonora and Sinaloa where cool moist conditions prevail during some years.
This merely underscores the role of specific environmental conditions in the erratic behavior of the disease. It also leads one to the conclusion that the disease could be introduced to an area and remain undetected until the optimum conditions for infection are present or until a highly efficient system is put in place to detect a small number of teliospores in field samples, conveyance systems or storage facilities.
Current Research
Current and recent research on Karnal bunt has focused on the biology and detection of the pathogen in seed lots with the teliospore being the focus of the detection system. Absolute identification of the pathogen is possible using the PCR system and several specific primers for the Tilletia indica teliospores. Detection systems are extremely efficient and are able, through a simple washing procedure to detect one spore in a 50 gram wheat sample. The system is so efficient that when spores are added to a clean 50 gram sample of wheat seed, 80 per cent can be recovered in the normal sampling procedure. This includes numbers as low as ten spores added where eight can be recovered. Soil detection is more difficult.
Research attempting to compare the significance of actual numbers of teliospores to the level of field infection has been done only in Mexico in the western hemisphere. In experimental plots in the Yaqui Valley, Sonora, wheat seed with 5, 10, 100, 250 and 500 infected seeds blended with a kilogram of clean seed were planted in replicated plots. It should be noted that at these levels of seed infestation, as many as one billion spores per kilogram may be introduced into each plot. During three years of the experiment in the Yaqui Valley, no treatment effect was noted. The highest level of infection during each of the three years of the experiment was either in the check in which only clean seed had been planted or the treatments with lower level of artificially infested seed. These relationships held even though the local disease level during the three years varied dramatically. The experiments were of necessity conducted on land already infested with Karnal bunt teliospores. The conclusion was that the key element in the level of infection is the high humidity, cloudiness and free moisture required for infection. Since the land was already infested, adding additional spores through infected seed did not influence the level of disease. There was no information as to the level of preexisting teliospores in the soil.
Dissemination of Karnal Bunt
As mentioned before, the chief means of dissemination of smut and bunt diseases is by infested seed. There seems nevertheless to be general agreement among Karnal bunt researchers that teliospores can be moved by windblown soil. Other types of fungal spores on the soil surface are routinely windblown. The most notable local parallel situation is the conidia of Aspergillus flavus, a fungus that colonizes cotton bolls in desert agriculture. These conidia have been isolated routinely in large quantities from windblown soil.
Karnal bunt spores have been found in Mexico near fields being harvested and particularly at high altitudes (3000 m) above fields in which the stubble has been burnt. At this height some standard calculations show that a spore the size and mass of a Karnal bunt teliospore could move in excess of 300 miles under appropriate wind conditions.
Other acknowledged means of dissemination of Karnal bunt spores include birds and insects. Both types of animals feed on infected grain with grasshoppers feeding preferentially on bunted grain. As the teliospores pass through the insects, germination is reduced by 20 to 50 per cent but the dissemination potential of the deposition of animal feces with viable teliospores, more than makes up for the reduction in the germination potential of the spores.
In discussing the dissemination of a disease like Karnal bunt, it is important to recognize the distinction between dissemination that occurs during annual disease development in an infested area and dissemination that introduces the disease to a new area. Based on the work done with Karnal bunt spores above burning wheat fields in Mexico it is apparent that movement over long distances (hundreds of miles) is possible. In addition to long range wind movement the infestations long known to exist in Mexico could also serve as a source of teliospores for long range movement in the produce industry. Infected wheat fields exist in areas where produce is grown and shipped to the U. S. via thousands of trucks each year. In the normal rotation of crops some former wheat fields will be planted to produce such as lettuce, celery, squash, tomatoes, cucumbers and peppers. Since most of these crops are packed in the field and are not washed, soil containing teliospores of Karnal bunt would adhere to the product, the packing boxes, trucks and move to all areas of the U. S.. This more than likely has been going on for twenty years or more.
There has been some discussion with reference to the fact that Karnal bunt teliospores do not withstand freezing like most smut spores. The evidence available on this point suggests the possibility that spores may withstand both prolonged periods of drying and freezing.
Karnal bunt was first detected in seed lots grown in Arizona during the 1995 season. It is highly likely that the pathogen was introduced at some time in the past, possibly years ago. It is probable that infection has always been low, making detection unlikely until the intense detection system was applied after a chance discovery of an infected grain on March 8, 1996. Such diseases ordinarily do not reach a detectable level until well established. Establishment of a disease of this type would probably take at least three years and probably longer. Because of this, it is likely that Karnal bunt is established in some desert agricultural areas at present and maybe other wheat production areas as well. It will not be possible to determine this until a rigorous sampling detection survey is conducted in other areas similar to that now underway in Arizona.
In addition, during the spring of 1995 there were unusual weather conditions in central Arizona that could have resulted in far more infection than usual. Evidence to support this is the fact that late blight of potatoes, caused by Phytophthora infestans, occurred in Central Arizona agricultural areas for the first time in at least 30 years. A late blight epidemic requires a prolonged period of cool, moist weather. Such conditions would have also encouraged the development of Karnal bunt to a greater incidence than normal in a desert agriculture area. The "hot spot" for KB is in the same general area where late blight was found in the Goodyear, Avondale area west of Phoenix. Checks of seed lots dating back at least to 1993 from the same area in central Arizona revealed the presence of Karnal bunt spores at low levels.
Diseases of wheat in the U.S.
Considering the relatively minor nature of the Karnal bunt disease in both its lack of significant impact on the crop yield and lack of any health hazard to humans or livestock, the level of concern that has been expressed by government officials and the reaction that is represented by an expensive and likely futile effort to eradicate the disease in Arizona and contiguous areas is surprising. This is particularly true because of the fact that it has almost surely been introduced in other areas despite negative but superficial surveys in some major wheat producing states.
Other long term wheat diseases are far more serious and some have serious health problems for both livestock and humans. There are 90 diseases listed in the compendium of wheat diseases published by the American Phytopathological Society. In the past, serious diseases of wheat have been relatively rare in Arizona, including Karnal bunt, while many diseases are serious problems in major wheat growing areas of the U. S. We will highlight several of the more important here and their general incidence in Arizona.
1. Black stem rust of wheat. (Puccinia graminis). This is the most serious from a crop loss standpoint in the U. S. heartland. It has been controlled for 80 years by an integrated program of cultural management, pathogen monitoring and genetic resistance. The disease is rare and of no importance in Arizona wheat production.
2. Ergot. (Claviceps purpurea). One of the worlds' classic wheat diseases. This disease is common on all grains. Though most serious on rye, it is a constant problem on wheat. Infected heads may have a number of black hard bodies (ergots) formed of fungal tissue in place of the grain. The ergots contain a series of poisons whose effects range from inducing abortions in both livestock and humans to interfering with circulation at the extremities resulting in gangrene and loss of limbs in severe cases. During the middle ages the bread made with ergot contaminated grain was responsible for some remarkable human suffering. The most recent serious epidemic with extensive human suffering was in the Ethiopian highlands in 1979. Because ergot is a problem in most parts of the world there are fairly high tolerances for grain contamination. The disease is not a problem in Arizona wheat.
3. Scab. (Fusarium graminearum). A disease that is a continuing problem in the Midwest. Infection of wheat results in the production of a series of toxins that have serious consequences when present in livestock feed. The most important toxin is Deoxynivalenol (= RD toxin = Vomitoxin). Scab does not occur in the main Arizona wheat production areas.
Quarantine Rationale
Routine movement of equipment and agriculture products of all kinds are likely methods, in addition to windblown soil and seed, of moving and introducing the Karnal bunt pathogen to a wide variety of locations both in the desert and other areas in the country. The nature of the Karnal bunt quarantine imposed by the USDA may suppress the disease, but it has no chance of eradicating the pathogen. The needle in a haystack efficiency of the detection plan in place will always be two steps behind the actual movement of the pathogen because it incorrectly assumes that the present infestation is new and therefore is confined to wheat fields. It cannot be determined for sure that other areas of the county are free of Karnal bunt until they are surveyed with the same rigor as Arizona.
Critical Research Needs
Far more important than the level of seed infestation is the population of teliospores in the soil. Those on or near the soil surface are the most important but there are no current efforts to focus on this question. It seems apparent, based on the Mexico research mentioned earlier in this document that the massive addition of teliospores via artificial seed infestation has no immediate impact on the level of disease. Research has shown also that a period of maturation is required for the spores to germinate. For the current season then the spores on the soil surface from previous years are those judged to be most critical to disease development and those for which a detection system is badly needed. The seed washing technique does not provide information that can be used to accurately asses the temporal disease potential due to the erratic nature of disease occurrence.
This report was prepared with the cooperation and input of the following College of Agriculture, University of Arizona scientists:
Robert L. Gilbertson, Professor of Plant Pathology
Merritt R. Nelson, Professor of Plant Pathology
Michael J. Ottman, Extension Specialist, Plant Sciences
Alan D. Simons, Research Scientist Plant Sciences
Michael E. Stanghellini, Professor of Plant Pathology
The contents of this statement on Karnal bunt are based on published literature, recent discussions with USDA and Mexican scientists, knowledge of the grain industry in the Sonoran desert and the general principles of plant pathology, including those that apply to the specialized areas of soil borne diseases, plant disease epidemiology and mycology.
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© Copyright 1996 by the American Phytopathological Society