Breakdown of the Yr17 resistance to yellow rust of wheat in northern Europe

The gene Yr17, conferring resistance to Puccinia striiformis f. sp. tritici , was introduced into northern European wheat cultivars in the mid 1970s. V irulence for Yr17 was not detected until 1994, when it was identified in th e UK and Denmark. In both countries, susceptible Yr17 cultivars were grown on an increasingly large area over the next four years and this was followe d by a rise in the frequency of corresponding virulence to 100%. Elsewhere in northern Europe Yr17 cultivars were grown on a relatively small scale du ring this period. However, Virulence for Yr17 was detected in France and Ge rmany in 1997 and reached a frequency in excess of 70% by 1999, suggesting an influx of pathogen spores from UK or Denmark. Migration of pathogen spor es around northern Europe clearly plays an important role in determining th e virulence composition of local populations of Puccinia striiformis f. sp. tritici. It is vital that this should be reflected in cooperation between national Virulence monitoring projects.