Assessing the Frequency of Spongospora subterranea f. sp. subterranea and Potato Mop-top Virus Occurrence in Seed Potato Lots Entering Oregon

Abstract Pathogens present in or on seed potatoes represent a risk for farmers because they can reduce the yield or quality of subsequent potato crops. However, data on the frequency that different pathogens occur in seed lots rarely exist for diseases that do not produce overt symptoms or signs as they are often not assessed by seed certification programs. From 2016 to 2025, we surveyed seed lots entering Oregon for the presence of Spongospora subterranea f. sp. subterranea ( Sss ), the causal agent of powdery scab, and potato mop-top virus (PMTV). Across nine years, we detected Sss , PMTV, and Sss and PMTV in 77.1%, 4.6%, and 5.3% of seed lots. While Sss was detected at high frequency across seed lots regardless of geographic origin or cultivar, PMTV exhibited sporadic, seed-lot–dependent detection with greater variability by state of origin and variety. Both pathogens were detected in seed lots lacking visible signs or symptoms. These data provide additional evidence that Sss and PMTV are being disseminated through seed, highlighting the need to adopt molecular diagnostics for assessing pathogen presence in seed potato lots.