Detection of Potato Mop-Top Virus in Potato Seed Lots Entering Washington State

For many years, potato seed lots have been tested for economically important pathogens in order to restrict their spread. Despite this, some pathogens inevitably make it into commercial fields and require management. Although knowledge about pathogen prevalence in commercial seed lots has increased over the last decade, assessment of the tuber necrotic viruses, Tobacco rattle virus and Potato mop-top virus, has been lacking. For seven years, four tubers from each seed lot in the Washington State University Seed Lot Trial were assessed for these two viruses. Tobacco rattle virus levels were negligible, but 1.76 to 5.50% of seed lots were infected with Potato mop-top virus each year. Potato mop-top virus was found in seven seed growing regions and 23 cultivars. These results are concerning, as the rate of transmission from seed to daughter tubers is largely unknown, and fields across the region are infested with Spongospora subterranea, the vector of Potato mop-top virus.