Potato virus Y (PVY) exists as a complex of strains, with multiple recombinants associated with tuber damage. Prevalence of strains was studied in the Idaho and Washington seed potato winter grow-out (WGO) trials, between 2016 to 2019. Foliar potato samples exhibiting mosaic during the WGO trials were collected and subjected to serological and molecular analyses using monoclonal antibodies and two sets of strain-specific primers probing most common recombinant junctions in PVY recombinants. In Idaho, prevalence of the non-recombinant strain PVY^O was stable at around 3%, while the most prevalent strain of PVY was found to be PVY^N-Wi, at approximately 70%, followed by PVY^NTNa, at around 20%, with little changes during the period of observation. In the 2019 season, the latter two recombinants, PVY^N-Wi and PVY^NTNa together, represented more than 90% of all PVY positives found in Idaho seed potato lots. In Washington, a drastic transition occurred, from mostly non-recombinant PVY^O and PVY^N strains in 2016, to mainly PVY^N-Wi and PVY^NTNa strains in 2019. The contrasting strain compositions of PVY in Idaho and Washington seed potato may reflect very different production practices between specialty potato cultivars grown in western Washington, and processing potato cultivars grown in Idaho.