Virulence characteristics of Bremia lactucae populations in Norway
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Use of resistant cultivars represent an efficient control measure for lettuce downy mildew (Bremia lactucae), although the durability of presently deployed resistance genes remains uncertain. Our objective was to document the pathogenic diversity of B. lactucae isolates in Norway. A total of 69 isolates of B. lactucae were collected between 2001 and 2006 from 65 commercial fields and four greenhouses in southeastern and southwestern Norway and tested for the presence of one or more of 19 virulence factors (v-factors). Phenotypic diversity was calculated based on presence or absence of v-factors, and as an overall comparison of v-phenotypes for each isolate. Disease severity varied over the years of the study, and epidemics were most consistently severe in southeastern Norway. The most commonly occurring v-factors, in order of frequency, were v5/8, v7, v2, v18, v4, v13, v6, v11, v12, v1 and v10. Virulence factor v17 was not found, while v36 was found in one isolate only. A total of 44 different v-phenotypes were identified within the population represented by the 69 isolates, yielding an incidence of unique virulence types of 63 %; a relatively high level of pathogen diversity. Four of the identified v-phenotypes were identical to races Bl:17, 18, 22 and 24, which have been previously reported in European populations of B. lactucae. The variability of the Norwegian B. lactucae populations verifies the genetic flexibility of this pathogen and its great ability to adapt to changes in host plants and surrounding conditions.
KeywordsEpidemiology Lactuca sativa Lettuce Lettuce downy mildew Race-specific resistance Virulence factors
We greatly appreciate the close cooperation with local advisors in the Norwegian Agricultural Extension Service who sampled the lettuce isolates. We also thank A. Hägnefelt, Swalöf Weibull, Sweden and I. Weber, Seminis Research, the Netherlands, and their colleagues for testing the B. lactucae isolates used in this study. Thanks also to T. Rafoss for statistical advice, and A. Stensvand for valuable comments on the manuscript. Figure 1 was kindly provided and adjusted by E. Fløistad. This study was part of a project funded by The Research Council of Norway, Innovation Norway and Norwegian lettuce growers.
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