Comparison of two aerobiological approaches for predicting the aerial incursion of soybean rust into the United States

Soybean rust (Phakopsora pachyrhizi) was first detected in Brazil in 2001 and is likely to be aerially transported into the U.S. The USDA-APHIS is funding a project to examine the threat of aerial incursion into the U.S. using two aerobiological models. The Integrated Aerobiology Modeling System (IAMS) uses the box-parcel method to predict spore movement and mortality. The HYSPLIT model uses a Lagrangian method to calculate air parcel trajectories and can provide complex dispersion and deposition simulations. Both models demonstrate that historical transoceanic movements of the fungus was feasible. The models are initialized with accurate source maps for South America and Africa. Both models showed that the aerial incursion of soybean rust is currently limited by a disconnect between when spores are produced in South American soybean fields and the availability of airflows to transport them across the intertropical convergence zone into the U.S. While the HYSPLIT model provides a useful method for case study analysis, the IAMS model had greater analytical capabilities. A qualitative risk assessment scheme using HYSPLIT has been used for 8 years to successfully forecast macro-scale downy mildew epidemics. Quantitative output from the IAMS model is being validated with the qualitative HYSPLIT modeling scheme using data from the 2004 downy mildew epidemic.