1. Department of Population Medicine and Diagnostic Sciences, College of Veterinary Medicine, Cornell University, Ithaca, New York
2. Present addresses: Laboratoire d'Ecologie des Hydrosystèmes Naturels et Anthropisés, Université de Lyon, France; and CNRS, Laboratoire d'Ecologie des Hydrosystèmes Naturels et Anthropisés, Université Lyon 1, Villeurbanne, France
3. Department of Biological Statistics and Computational Biology, Cornell University, Ithaca, New York
4. Present address: Department of Plant Pathology and Plant-Microbe Biology, Cornell University, Ithaca, New York