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Virulence of North American and European Isolates of Verticillium albo-atrum on Alfalfa Cultivars. A. A. Christen, Research associate, Department of Plant Pathology, Washington State University; R. N. Peaden(2), G. P. Harris(3), and J. B. Heale(4). (2)Research agronomist, ARS, USDA, Irrigated Agriculture Research and Extension Center, Prosser, WA 99350; (3)Postgraduate research student, Department of Biology, Queen Elizabeth College, University of London, Campden Hill, London W8 7AH, U.K.; (4)Senior lecturer, Department of Biology, Queen Elizabeth College, University of London, Campden Hill, London W8 7AH, U.K. Phytopathology 73:1051-1054. Accepted for publication 5 January 1983. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1983. DOI: 10.1094/Phyto-73-1051.

Four North American and three European geographical isolates of Verticillium albo-atrum from Medicago sativa were used to inoculate two U.S. and four European cultivars of M. sativa. Tests at two temperatures were conducted at London, U.K., and at Prosser, WA. Incubation temperatures at London were fluctuating, 17- 30 C (night- day) and 12- 21 C, and at Prosser were 27 ± 2 C and 20 ± 1 C, respectively. Disease severity was significantly greater (P = 0.01 and 0.05, respectively) at the higher temperatures at both locations. Differences in percent resistant plants between each of the cultivars, Maris Kabul, Vertus, Europe, Sabilt, Agate, and Apalachee, were significant (P = 0.01) except that Agate and Apalachee were not different at London, and Sabilt was not included in the Prosser test. Relative rank order of the cultivars, based on mean percent resistant plants, was the same at both locations with a high of 80% for Maris Kabul and a low of 1% for Apalachee. The temperature x isolate interaction was not statistically significant, but the temperature x cultivar interaction was significant (P = 0.01). A significant (P = 0.05) mean difference of 1.6% resistant plants following inoculation of alfalfa with European and North American isolates at Prosser was not supported by the data obtained at London. This difference is smaller than is usually obtained in tests of a single isolate in repeated tests with the same cultivars. We concluded that there is no difference among isolates as measured by virulence on highly susceptible and highly resistant alfalfa cultivars and that the North American outbreaks of Verticillium wilt very likely originated from introduction of a European strain of the pathogen.