1. U. S. Department of Agriculture, Agricultural Research Service, Western Regional Research Center, 800 Buchanan Street, Albany, CA 94710-1105.
2. Corresponding author. Phone: 510/559-5687. E-mail: kasarda@pw.usda.gov
3. Dipartimento di Agrobiologia e Agrochimica, Università degli Studi della Tuscia, Via S. Camillo de Lellis, 0100 Viterbo, Italy.