Thesis (Ph.D., Plant, Soil and Entomological Sciences) -- University of Idaho, December 2014 | Potato virus Y (PVY) and Potato leaf roll virus (PLRV) constrain potato production and are transmitted by many aphid vectors. Green peach aphid (GPA) is the most efficient vector of both viruses. The overall objective of this dissertation was to study the potato-PVY- and PLRV-aphid pathosystems. A recent concern regarding PVY is the emergence of necrotic and recombinant strains (e.g., PVYN:O, PVYNTN, PVYNA-N/NTN) and their impacts relative to the ordinary strain (PVYO) in the United States. The first study was aimed at clarifying transmission of virus strains by GPA when multiple virus strains are present in the same source tissue. The apparent primacy of PVYO observed in the study suggests that GPA transmission from mixed infection does not contribute to the increasing prevalence of newer necrotic strains. The second study was aimed at understanding how necrotic PVY isolates are transmitted compared to ordinary isolates by various colonizing (GPA and potato aphid, PA) and non-colonizing aphid (bird cherry-oat aphid, BCOA) species. Although GPA transmitted PVY isolates most efficiently, BCOA transmitted PVYNTN isolates with better efficiency than previously reported. BCOA is one of the most abundant aphids in potato fields in Idaho, suggesting that this species might be a contributing factor to the recent prevalence of necrotic strains. The third study was focused on the characterization of the aphid species complex over time in potato fields adjacent to cereal fields and how their transient flight might contribute to PVY incidence. A diverse fauna of non-colonizing aphid species was captured, including many cereal aphids as well as species from other crops and weeds. PVY incidence in potato increased following peak aphid flight and appeared to be related to aphid abundance. The objectives of the PLRV-aphid pathosystem study were to observe the phenology of aphid vectors and PLRV incidence among Russet Burbank, Russet Norkotah, and Ranger Russet potato varieties over the season. All varieties were found to be at similar risk of PLRV incidence and aphid vector colonization. Findings from these studies contribute to our understanding of the PVY- and PLRV-pathosystems, and their future management for the benefit of potato growers.