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Ability of Aphanomyces euteiches to Cause Disease of Seedling Alfalfa Compared with Phytophthora megasperma f. sp. medicaginis. E. B. Holub, Former graduate research assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706; C. R. Grau, professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 80:331-335. Accepted for publication 22 September 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-331.

Four-day-old alfalfa seedlings were inoculated with zoospores of Aphanomyces euteiches or Phytophthora megasperma f. sp. medicaginis and incubated at 16, 20, 24, 28, or 32 C. Seedling survival was recorded at 3, 6, and 10 days after inoculation, and disease severity was rated on a four-class scale 10 days after inoculation. Both pathogens caused root discoloration and chlorosis of cotyledons, followed by advancing stem necrosis. Disease incidence was equivalent for seedlings incubated at 16?28 C. However, optimal temperatures for disease, measured by disease severity rating and area under a mortality progress curve, were 24?28 C and 16?28 C for A. euteiches and P. m. medicaginis, respectively. The reaction of alfalfa cultivars Vernal and Apollo II also was examined. Both cultivars are susceptible to A. euteiches, and Apollo II is resistant to P. m. medicaginis. Vernal reacted with higher disease ratings and greater area under a mortality progress curve than Apollo II when inoculated with either pathogen. However, the difference between cultivars was greatest against P. m. medicaginis at 16 and 20 C.