A model of the escape of Sclerotinia sclerotiorum ascospores from pasture

A multi-layer physical model, sporesim-1d, 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-1dʹs context is the risk analysis of S. sclerotiorum used as a mycoherbicide to control Cirsium arvense in pasture. In validation tests sporesim-1d 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-1d, were 50% lower than predicted by both a Lagrangian model (Plant Disease 82 (1998) 838) and a one-layer version of sporesim-1d, sporesim-1l, indicating that the vertical compartmentalisation in sporesim-1d, 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-1d 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 (r2=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.