1. a EARS Ltd, Remote Sensing Consultants , P. O. Box 449, Delft, EB, 2628, The Netherlands
2. b Institute of Research on Electromagnetic Waves (IROE), National Research Centre (CNR) , Firenze, Italy
3. c Department of Plant Physiological Research , Agricultural University of Wageningen , Wageningen, The Netherlands
4. d Institute of Plant Disease Research , Wageningen, The Netherlands