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A Screening Technique Useful in Selecting for Resistance in Alfalfa to Phytophthora megasperma. F. A. Gray, Research Assistant, Department of Plant Pathology, University of Arizona, Tucson 85721; R. B. Hine(2), M. H. Schonhorst(3), and J. D. Naik(4). (2)(4)Professor, and former graduate student, respectively, Department of Plant Pathology, University of Arizona, Tucson 85721; (3)Professor, Department of Agronomy and Plant Genetics, University of Arizona, Tucson 85721. Phytopathology 63:1185-1188. Accepted for publication 27 March 1973. DOI: 10.1094/Phyto-63-1185.

A screening technique, consisting of planting alfalfa seed in infested soil, was developed for selecting plants with resistance to damping-off and root rot caused by Phytophthora megasperma. A high level of resistance to damping-off was obtained in the nonwinter-hardy alfalfa cultivar ‘Hayden’ after one cycle of selection. The amount of pre- and post-emergence damping-off was influenced by the inoculation technique. High pre-emergence loss occurred when the inoculum was placed with the seed before covering with a heat-pasteurized sand-soil mixture; whereas, high post-emergence loss occurred when the inoculum was blended into the sand-soil mixture before seeding. Seedling disease increased in direct proportion to inoculum concentration. When seed of the cultivar Hayden were planted in flats containing previously infested soil, a 98% seedling stand loss occurred after 2 weeks incubation in growth chambers with a 12-hr light cycle at 24 C and 12-hr dark cycle at 18 C. Ten isolates of P. megasperma recovered from diseased taproots of alfalfa from different geographic areas in Arizona, one isolate from California, and two isolates from Minnesota, were pathogenic to alfalfa grown in the greenhouse in Arizona.

Additional keywords: oospores.