Impact of a maize buffer on turbulent dispersion and deposition of Phytophthora infestans spores

For many agricultural pests and diseases (e.g. potato late blight caused by Phytophthora infestans), airborne transport is an important step in their spread. Hence, understanding and possible control of this transport is essential in controlling the spread of diseases. In addition, pesticides, used to control agricultural pests and diseases , are evaluated in registration trials prior to admission. In the Netherlands protocols for this type of field experiments include the obligation to surround the trials with a 4.5m wide buffer, usually maize, to prevent or limit P. infestans spore dispersal to neighbouring fields.

The objective of this study is to determine how this maize buffer influences P. infestans spore deposition behind the buffer under different atmospheric conditions. In this study, the Dutch Atmospheric Large-Eddy Simulation model (DALES), recently extended with a description of canopy drag, is used for high-resolution simulations of the atmospheric surface layer, including the canopy. These simulations are used to determine space- and time-dependent velocity fields. As spore release usually appears around sunrise, simulations of slightly stable, neutral and slightly unstable conditions were performed. The LES fields are used as input for a Lagrangian stochastic model (LSM), which determines trajectories of individual spores through the turbulent atmospheric surface layer. The LSM includes the deposition of spores on the crop (either potato or maize). Observations from literature are used to validate the combination of LES and LSM as a tool to study spore dispersion.

First of all, results show that a maize buffer does have an effect on spore dispersion and deposition patterns. This effect is mainly caused by modification of flow patterns, such as reduction of the wind speed in and behind the buffer and the presence of a recirculation zone directly behind the buffer. Second, the size of the trial field affects the impact of the buffer on dispersion and deposition characteristics. These results indicate that field trials should not be too large. Finally, it was shown that for unstable conditions the effect of the maize buffer on dispersion and deposition patterns is smaller. The findings of this study suggest that the maize buffer does reduce the number of spores to be transported and deposited on neighbouring fields. However, it is difficult to translate this to a true reduction of potato late blight infections, because information on the exact amount of released spores is needed to make this assessment.

Recently, a small field experiment, has been performed. Data of this field experiment are used to validate the combined LES-LSM with observations. During this experiment potato crops were infected with P. infestans to have a clear spore source. Additionally, experiments using controlled release of Lycopodium spores were performed. Observations included spore concentrations upwind and downwind of the maize buffer as well as meteorological observations.