Yellow rust epidemics, caused by Puccinia striiformis West., often arise in wheat field as infection focus of a few decimetres of diameter, including some sporulating lesions on a few plants. Under appropriate environmental conditions such as high relative humidity and temperatures between 2 and 20 degrees C, those initial foci may grow rapidly and initiate new foci elsewhere in the field. The first aim of this work was to better understand the effects of weather parameters on the focus growth. An experiment was conducted in a wheat field during the 2001 season to measure, from inoculated plants, the disease progression related to climatic conditions (temperature, relative humidity, precipitations and wind). Three plots were inoculated in March and the sporulating lesions around each focus were monitored every week on 8 segments starting from the centre of the plot, by recording the infected leaf layers and the spatial position of every infected plant. Once established, the disease spread not only horizontally, by spore transport from plants to plants, but also vertically, by spore dispersal from lower leaf layers to upper ones. The focus required a build up period, with diseased plants confined to a circle of maximum 3 m diameter around the centre with the inoculated plants, before a widespread expansion. This initial build up period required at least two generations. On base of the changes of the disease status observed every week and the calculation of the latent period, the supposed infection dates and the environmental factors responsible for those infections were determined. This allowed adjustment of an infection forecasting model based on weather data. These results will be integrated into a decision support system to control the disease before the occurrence of large scale inoculum dispersion.