the fate of particles released in a canopy as a function of canopy and particle characteristics

Fungal diseases, such as downy mildews and wheat rusts, routinely threaten field crops. Integrated pest management strategies seek to limit fungicide use by monitoring outbreaks and forecasting the transport of fungal spores, so that chemical applications may be timed to the stage of infection when the pathogen is most vulnerable. Models of crop infection by fungal diseases must track the life cycle of fungal spores through release, escape from the canopy, transport and loss of viability in the atmosphere, and deposition on new host surfaces. In this paper, we focus on the process of spore escape from the canopy. Current infection prediction systems, such as the Integrated Aerobiology Monitoring System (IAMS), describe the fraction of produced spores leaving the canopy (spore escape fraction) with a wind speed dependent formulation. However, the following additional factors are expected to influence escape fraction: height of release within the canopy, particle settling velocity, and canopy density. A simple random walk particle tracking (RWPT) model was created to explore these factors. The model is validated by comparison to large eddy simulations of Pan, Chamecki, and Isard over identical model cornfields. Particle escape fraction and plume evolution above the canopy agree within uncertainty, showing that an eddy diffusivity approach is effective for predicting escape fraction from a single point source of particles inside the canopy. The agreement suggests that the inclusion of turbulence persistence, or periods of sustained positive or negative turbulent velocity associated with coherent turbulent structures at the canopy interface, is not critical in modeling particle escape fraction. That is, simulation of vertical diffusion via a simple random walk produces similar results to simulation using a velocity field with persistent events. This effect is explained through consideration of the velocity integral time scale and the particle escape timescales. Once validated, the model is used to explore escape fraction behavior for a range of canopy and spore characteristics that mimic field conditions. The settling velocity of disease spores and pollen grains in still air ranges from 0.2-2 cm/s (Gregory, 1961). In the field, canopy shear velocity values, a measure of turbulent velocity scales, ranges from 10 to 100 cm/s, yielding a ratio of spore settling velocity to canopy shear velocity of 0.002-0.2. The ratio of canopy drag length scale to canopy height of wheat, soybeans, and corn varies between 0.47-2.5 under common planting schemes (Wilson et al., 1982; Baldocchi et al., 1983; Brunet et al., 1994). Spore release height often starts from an initial low position and approaches the canopy interface as the epidemic progresses. In addition to presenting modeling trends across the full parameter space, we also compare model particle behavior to a specific set of field observations describing the 1982 tobacco epidemic (P. tabacina), recorded by Rotem and Aylor.