A model of the escape of Sclerotinia sclerotiorum ascospores from pasture

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A multi-layer physical model, sporesim-1 d, based on the gradient transfer theory (K-theory) of turbulent dispersal (analogous with the molecular diffusion of gasses) is described for the transport of Sclerotinia sclerotiorum ascospores within and above a grass canopy following their release from apothecia at ground level. The ‘steady-state’ diffusion equation is solved numerically and the spore escape fraction is estimated. sporesim-1 d's context is the risk analysis of S. sclerotiorum used as a mycoherbicide to control Cirsium arvense in pasture. In validation tests sporesim-1 d was internally consistent and produced a vertical wind speed profile similar to that measured in a grassland. In further validation tests, measured vertical profiles of atmospheric concentrations of Lycopodium clavatum spores in a wheat crop, and Venturia inaequalis spores in an apple orchard and in a grassland, were closely approximated by the model, as was measured data on the concentration of S. sclerotiorum ascospores deposited downwind of a small area source in a grassland. Escape fractions for grassland predicted by sporesim-1 d, were 50% lower than predicted by both a Lagrangian model (Plant Disease 82 (1998) 838) and a one-layer version of sporesim-1 d, sporesim-1 l, indicating that the vertical compartmentalisation in sporesim-1 d, allowing wind speed and pasture leaf area index (LAI) to vary with height, results in a more realistic estimate of the escape fraction. Simulations using sporesim-1 d revealed an increase in the escape fraction with increasing wind speed, and an order-of-magnitude fall with increases in LAI from values typical of a closely grazed sheep pasture (ca. 2) to those of more laxly grazed cattle pastures and intact grassland (ca. 7). This result implies that any additional risk of disease in a susceptible crop growing downwind of a pasture treated with a S. sclerotiorum mycoherbicide may be reduced by grazing management. Reduction in the risk of sclerotinia rot in kiwifruit ( Actinidia deliciosa) vines, and in apple scab disease in apple trees, caused by V. inaequalis, appears possible by maintaining a dense grass under-storey. A simple empirical model for spore escape with one parameter and two variables (LAI and wind speed) derived from the mechanistic model provided a good description ( r 2=0.998) of simulated escape fraction. Combined with information on release rates of S. sclerotiorum spores at a biocontrol site, this model will enable a times-series analysis of spore emission, and coupled with a Gaussian plume model, prediction of minimum isolation distances between a biocontrol site and a susceptible crop.