Virulence of North American and European Isolates ofVerticillium albo-atrumon Alfalfa Cultivars

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Abstract
Four North American and 3 European geographical isolates of V. albo-atrum from M. sativa were used to inoculate 2 USA and 4 European cultivars of M. sativa. Tests at 2 temperatures were conducted at London, UK and at Prosser, Washington [USA]. Incubation temperatures at London were fluctuating, 17.degree.-30.degree. C (night-day) and 12.degree.-21.degree. C, and at Prosser were 27.degree. .+-. 2.degree. C and 20.degree. .+-. 1.degree. 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% of ''Maris Kabul'' and a low of 1% for ''Apalachee''. The temperature .times. isolate interaction was not statistically significant, but the temperature .times. 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. 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.

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