1. Department of Plant and Environmental Sciences, Norwegian University of Life Sciences, Ås, Norway.
2. Corresponding author. Phone: +47 45521244. Fax: +47 64965001. E-mail: selag@umb.no
3. Department of Plant Pathology, University of Minnesota, Saint Paul, MN, U.S.A.
4. Nofima AS, Osloveien 1, NO-1430 Ås, Norway.