Genetic structure and pathogenicity of populations of phytophthora infestans from organic potato crops in france, norway, Switzerland and the United Kingdom - Flier, W. G; Kroon, L. P. N. M; Hermansen, A; Van Raaij, H. M. G; Speiser, B; Tamm, L; Fuchs, J.G; Lambion, J; Razzaghian, J; Andrivon, D; Wilcockson, S; Leifert, C