From lesion to region : epidemiology and management of potato late blight

Keywords: Infection pressure, Phytophthora infestans , inoculum sources, modelling, spore dispersal.

Aerial dispersal of Phytophthora infestans spores from distant sources to crops is an essential part of the epidemiology of potato late blight. This makes late blight a regional problem. An interdisciplinary analysis of the regional late blight problem is carried out through model development, experimental parameterisation and analysis and scenario studies that investigate possibilities for effective control of the disease at the regional level.A new equation was derived to estimate the relative exponential growth rate r (d^-1) of a plant disease epidemic from commonly used component parameters for pathogen aggressiveness and host resistance, such as the latency period, infection efficiency, sporulation intensity and lesion growth rate. The use of the equation is demonstrated with field measurements of resistance components against late blight for five potato cultivars. Infection efficiency and lesion growth rate together explained most of the variation in cultivar resistance.To describe the dispersal of spores at distances up to 10 km downwind from a source of inoculum, the Gaussian plume model was used. A field experiment was set up to calibrate the Gaussian plume model, as applied to the dispersal of spores. A comparison of estimated concentrations with the measurements confirmed that spore clouds originating from a point source take the form of a Gaussian plume: the coefficient of correlation between measured spore concentrations and fitted concentrations was 0.8. The fraction of spores that escaped the canopy and was available for long distance dispersal amounted to 64%±17%. : exactly'>To model deposition and loss of spores from the spore plume at distances between 50 m and 10 km from the source, the source depletion method was used. This is a practical method, but it is simplified in its description of spore loss. The accuracy of the source depletion method was determined by comparing it with the more realistic surface depletion method in a modelling study. It was found that under worst case conditions, the source depletion method may lead to an error of at most a factor 4 in calculated deposition of Phytophthora infestans spores.The infection pressure on receptor crops caused by inoculum from a distant source was calculated with a newly developed model. The sensitivity analysis showed that disease level at the source had by far the greatest impact on infection pressure, followed by distance from the source. Subsequent scenario studies indicated that eradication of sources with high disease levels and spatial separation of cropping systems with different disease tolerances are more effective than use of more resistant cultivars for the receptor crop or a ban on the growing of susceptible cultivars. The conditions and possibilities for practical implementation of the effective control strategies as well as their consequences for fungicide requirements are discussed.