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Factors Involved with the Reaction of Alfalfa to Root Rot Caused by Phytophthora megasperma. G. C. Marks, Postdoctoral Fellow, University of Wisconsin, Madison 53706, Present address of senior author: Senior Forest Pathologist, Forests Commission of Victoria, Melbourne, Australia; J. E. Mitchell, Professor of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 61:510-514. Accepted for publication 24 November 1970. DOI: 10.1094/Phyto-61-510.

The reaction of alfalfa to root rot caused by P. megasperma was examined by comparing the response of susceptible and tolerant varieties and selections to infection in solution culture and in soil. Considerably less root rot occurred in highly tolerant than in susceptible plants. Zoospores of P. megasperma encysted readily on both susceptible and tolerant plants. Resistance to infection in growing root tips was associated with a hypersensitive type of reaction to infection of young cortical cells. Roots with large-diameter central steles survived better in infested soil. Tolerant selections produced more lateral roots with large-diameter central steles than did those susceptible to root rot.