1. a Horticulture Research International, Crop Science, Perennial Crops Division , East Mailing, Kent, ME19 6BJ, UK
2. c Horticultural Research Institute of Ontario, Ontario Ministry of Agriculture and Food , Simcoe, Ontario, Canada , N3Y 4N5
3. b The Norwegian State Agricultural Research Stations, Ullensvang Research Station , Lofthus, N-5774, Norway