1. Plant Research Department, Risø National Laboratory, PO Box 49, DK‐4000 Roskilde, Denmark
2. Department of Microbiology, Biological Institute, University of Copenhagen, Øster Farimagsgade 2D, DK‐1353, Denmark
3. Agricultural Research Centre of Finland–MTT, Laukaa Research and Elite Plant Station, Antinniementie 1, FIN‐41330 Vihtavuori, Finland