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A Risk Assessment Model for Importation of United States Milling Wheat Containing Tilletia contraversa

Dwarf bunt of wheat, caused by the fungus Tilletia contraversa, is a pathogen historically limited in distribution by its very specific climatic requirements for establishment. In an effort to both address the need for adequate protection and eliminate unwarranted trade barriers, a number of countries have examined restrictions on importing milling wheat containing teliospores of T. contraversa. Pest risk analysis (PRA), under the guidelines of the World Trade Organization and Food and Agriculture Organization, has become an internationally accepted process for evaluating such issues. As a component of a dwarf bunt PRA, our objective was to develop a quantitative mathematical model to evaluate and communicate the potential risk of dwarf bunt establishment from the importation of U.S. milling wheat containing teliospores of T. contraversa. A T. contraversa--risk model (TCK-risk model) was developed using new data, historic literature, and industry statistics to estimate teliospore diversion from transport and milling processes, spore contamination levels, grain handling, and end-product usage. A climatic model was developed to identify potential regions where the environment was favorable for disease development. The likelihood of disease establishment and wheat yield loss was determined using the model to conduct Monte Carlo simulations of 100,000 wheat shipping-years. The model is dynamic in that individual components can be easily updated or modified in order to determine the effect of numerous scenarios (e.g., climate changes, shipping tonnage, contamination levels, mitigation strategies) on the probability of dwarf bunt establishment. TCK-risk model evaluation scenarios previously conducted for the People's Republic of China, Brazil, Mexico, and Peru are presented as examples.