Aerial spore dispersal close to a focus of disease

Abstract

Horizontal fluxes of spores close to a source were examined using observations of spore concentration and wind speed above bare ground, and in and above a 1-m tall wheat canopy. These fluxes were estimated using a Markov-chain random-flight simulation model. For spores released over bare ground, the peak flux occurred at the height of the source and the vertical spread of the spore plume was described well by using a Lagrangian length scale L=0.5 z in the simulation model, where z is height above the ground. For spores released inside the canopy, the peak flux, at a downwind distance of 2 m, occurred well above the canopy. Reasonable agreement between the model calculations and the observations was obtained by setting the Lagrangian time scale equal to 0.5 h/ σ w ( h) inside the canopy, where h is the height of the canopy and σ w is the standard deviation of the vertical velocity fluctuations. For the wheat canopy used in this study, L at the top of the canopy was ∼0.5 m, which is about five times larger than expected from average momentum transport derived from the vertical profile of average wind speed measured above the canopy. The enhanced values of the mixing length for vertical transport of spores near the source are no doubt due to large-scale coherent motions in the flow, which favor escape of spores from the canopy over their filtration by plants.