Modeling local to regional movement and dry deposition of grass stem rust urediniospores with CALPUFF

Dispersal and dry deposition of the urediniospores of Puccinia graminis subsp. graminicola from infected perennial ryegrass in the Willamette Valley of Oregon was modeled with CALPUFF, a Lagrangian puff atmospheric dispersal model developed for air pollution studies. The emission rate was back-calculated from the measured aerial spore flux immediately downwind from a 6 X 6 m source plot located in a larger field of non- host plants with a canopy height and structure similar to the ryegrass. The diurnal pattern of spore release was obtained from volumetric spore sampler measurements in a nearby field. Meteorological observations at the study site were combined with three-dimensional prognostic meteorological fields generated by the Penn State Mesoscale Meteorological Model (MM5) using CALMET. CALMET adjusted the meteorological fields based upon the local topography, land use, and other weather monitoring stations in the region. The settling velocities of single and clustered spores were measured in the laboratory. Deposition rates and mass balances were modeled by using for four case studies in June 2005. Model results are presented for the 6 x 6 m source release, as well as a range of hypothetical grass field sizes, typical of the region. In addition, to the grass canopy release scenarios, results of a hypothetical dust devil, in which grass seeds are lifted up to 30 meters above the ground, are also presented.