The epidemiology of Phytophthora infestans (P.i.) in potato has changed dramatically because of sexual reproduction that became possible after introduction of both pathogen mating types in Europe in the late 1970’s. From 1845 untill this period only one m ating type was present in Europe. In the Nordic countries, the evidence for a change in the P.i. epidemiological behaviour has become more and more clear during the period 1997-2001. The existing late blight forecasting models and decision support syste ms (DSS) in the Nordic countries (and most other EU countries) are based on algorithms for pathogen behavior developed on experimental data from the 1950’s and 1960’ s. These data are is not reliable any longer. At the same time political action plans in the Nordic countries demands potato growers to minimise the pesticide use. To reach this goal it is vital to update existing decision support system with new biological data. The objective of this project is to evaluate and quantify epidemiological parameters of the ”new” population (s) of Phytophthora infestans in the Nordic countries. Obtained results will be implemented in existing and new forecasting - and decision support systems for late blight control. During the project a common framework for development of a core Internet based late blight decision support systems will be developed. This framework will secure that future development of late blight DSS applications and components can be exchanged between partner countries.