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Resistance to Phytophthora Root Rot in Selected Lines of Nondormant Alfalfa. R. B. Hine, Professor, Department of Plant Pathology, University of Arizona, Tucson 85721; F. A. Gray(2), M. H. Schonhorst(3), and J. S. Sanders(4). (2)(4)Research Assistants, respectively, Department of Plant Pathology, University of Arizona, Tucson 85721; (3)Professor, Department of Agronomy and Plant Genetics, University of Arizona, Tucson 85721. Phytopathology 65:840-844. Accepted for publication 22 February 1975. DOI: 10.1094/Phyto-65-840.

A high level of field resistance in alfalfa to root rot caused by Phytophthora megasperma was achieved in selections from a seedling screening of the nondormant alfalfa cultivar Hayden. In several greenhouse and field trials, a second-cycle polycross (Hayden PX II) had less root rot and higher stand density and plant vigor than a first-cycle polycross (Hayden PX I) or Lahontan, a semi-dormant cultivar with known resistance to root rot. In one field trial after 8.5 months of growth in infested soil, the percentage of disease-free plants was: Hayden PX II, 84; Hayden PX I modified, 62; Lahontan, 56; and Hayden, the cultivar from which the selections had been made, 28. In another field test, Hayden PX II, Hayden PX I, and Hayden had 67, 41, and 15% disease-free plants, respectively. The resistant synthetics, Hayden PX I and II, originally selected after seedling greenhouse exposure to a mixture of isolates from central Arizona, were equally resistant to other isolates from widely separated geographical areas in Arizona.

Additional keywords: Phytophthora megasperma, Medicago sativa L., breeding for disease resistance.