Use of a complex air pollution model to estimate dispersal and deposition of grass stem rust urediniospores at landscape scale

Dispersal and dry deposition of urediniospores of Puccinia graminis subsp. graminicola from infected perennial ryegrass was modeled with CALPUFF, a Lagrangian puff atmospheric dispersal model developed for air pollution studies. The emission rate (spores/min/m2 field area) was back-calculated from aerial spore flux from a 6 m × 6 m source plot. Spore flux was measured by a novel approach in which the cross-section of the entire plume was sampled immediately downwind of the source. The diurnal pattern of spore release was obtained from continuous volumetric spore sampler measurements in a nearby field. Weather observations at the study site were combined with three-dimensional prognostic meteorological fields generated by the Penn State Mesoscale Meteorological Model (MM5) using CALMET to run CALPUFF. The settling velocities of single and clustered spores were measured in the laboratory. Deposition fields and mass balances from the 6 m × 6 m plots were modeled by using observations from four dates in June 2005. A deposition level of 10 spores/m2 was estimated to reach 1.8–2.1 km downwind of the plot on different days. For three of the dates CALPUFF estimated few or no spores remaining airborne as long as an hour after release above the canopy. Modeling of spore dispersal from 5- or 50-ha fields of rust-infested perennial ryegrass indicated deposition of 10 spores/m2 at a distance of 2.1–5.9 km from the field, depending on weather and source strength (size of field and severity of rust disease), and deposition of 0.01 spores/m2 was indicated to extend more than 12 km from a heavily infested 50-ha field for one of the days. In this scenario 1014 spores were released from the field in 24 h, 18 million remained airborne more than an hour and 400,000 reach the air aloft. Simulation of spore dispersal by a small dust devil showed 12.5% of the spore biomass advected horizontally beyond 200 km in the air aloft.