1. Dipartimento di Protezione delle Piante e Microbiologia Applicata, Università degli Studi and Istituto di Virologia vegetale CNR, Sezione di Bari, 70126 Bari, Italy;
2. Plant-Pathogen Interactions Division, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, United Kingdom;
3. Germplasm Enhancement and Crop Improvement Division, International Potato Center, Lima 12, Peru and 4Department of Plant Biology and Forest Genetics, Swedish University of Agricultural Sciences, 750 07 Uppsala, Sweden;
4. Department of Botany and Plant Pathology and Center for Genome Research and Biocomputing, Oregon State University, Corvallis, Oregon 97331-2902;