Alfalfa Mosaic Virus (AMV, genus Alfamovirus)
Not a common virus in potatoes from what I can tell. It is one of those that turns up from time to time in well-traveled potatoes. ‘Black Irish’ and ‘Peruanita’ both tested positive here for AMV. The primary symptom is known as calico, which is essentially a very pronounced sort of mosaic. There may also be some tuber necrosis, beginning at the proximal end and spreading in storage. Primarily spread by aphids and also by contact or mechanical means. It may also be asymptomatic. A small amount of transmission though seed has been observed in some potato relatives, so it seems likely that this virus infects TPS to some degree. Tubers are infected unreliably, so it may be possible to recover disease free tubers from infected plants. Field tests are available. Not worth screening unless you know it is in your collection. You are at greater risk if you grow potatoes near alfalfa, since alfalfa tends to be heavily infected, even through seed.
More info: DPVWeb, EPPO, Ephytia (Pictures), Cropwatch (Pictures), Plantwise