Rain-splash pathogen dispersal: some results from leaf to regional scale

When a raindrop hits a plant surface, it can break into splash droplets. These one can carry pathogen agents growing on plant surface and disperse pathogens at short distances (around 1 m in free space) to nearby plant organs (Fitt and McCartney, 1986; Huber et al., 2006; Madden, 1992). This mechanism is of great importance for a number of fungal plant diseases of major crop ecosystems such as Septoria or Fusarium head blight of wheat. We will present here, the several aspects of the rain splash dispersal of plant pathogen, from the leaf to the regional scale.

First, based on previous work at leaf scale, we examine the potential of using physical parameters such as kinetic energy or impact force of impacting water drops of given parameters for predicting the pathogen dispersal from leaf lesions. (Saint-Jean et al., 2006)

Second using a Monte Carlo integration approach to simulate water transfer by rain-splash in a 3D canopy (Saint-Jean et al., 2004), we show how the dispersal of splash droplets at leaf scale can be integrated at a canopy scale described in 3D. We investigate several spatial organizations of the canopy structure and rainfall parameters such as raindrop size distribution and terminal velocity to determine scale dispersal of the rain splash. A sensitivity analysis of this modelling approach will be illustrated.

Third, it is shown how to estimate the splash potential at regional scale through integral rain parameters such as rainfall rate, kinetic energy, and rainfall power. A bipolar radar technology (Hydrix®, Novimet SA) provide the rainfall rate, information on raindrop concentration with time (2'30'') and space (1km²) resolutions. According to Testud et al. (2001), statistical parameters of the rain drop size distribution law is computed and Integral Parameters (rainfall rate, power, etc.) deduced. Because this methodology allows to characterize the precipitation field, it provides a real opportunity to identify most suitable integral parameters to predict the risk of splash-dispersal of at local scale. The long-term objective of this work is to contribute to the risk assessment of splash-dispersed of fungal diseases.

Fitt, B.D.L. and McCartney, H.A., 1986. Spore dispersal in splash droplets. In: P.G. Ayres and L. Boddy (Editors). Cambridge University Press.

Huber, L., Madden, L.V. and Fitt, B.D.L., 2006. The epidemiology of plant diseases. In: B.M. Cooke, D.G. Gareth and B. Kaye (Editors). Springer.

Madden, L.V., 1992. Rainfall and the dispersal of fungal spores, Advances in Plant Pathology, pp. 40-79.

Saint-Jean, S., Chelle, M. and Huber, L., 2004. Modelling water transfer by rain-splash in a 3D canopy using Monte Carlo integration, Agricultural and Forest Meteorology, pp. 183-196.

Saint-Jean, S., Testa, A., Madden, L.V. and Huber, L., 2006. Relationship between pathogen splash dispersal gradient and Weber number of impacting drop, Agricultural and Forest Meteorology, pp. 257-262.

Testud, J., Oury, S., Black, R.A., Amayenc, P. and Dou, X., 2001. The concept of "normalized" distribution to describe raindrop spectra: a tool for cloud physics and cloud remote sensing, Journal of Applied Meteorology, pp. 1118-1140.