Symptoms caused by Potato Virus Y (PVY) vary quite extensively in appearance and also severity, from negligible to severe. This is due to the strain (type) of PVY causing the infection and the potato variety. PVYNTN is now the dominant of the 3 PVY strains occurring in the USA. Its presence has resulted in this disease re-emerging to become a serious problem in the USA. This is partly because PVY is difficult to detect in many varieties because of very mild symptoms. Consequently PVY is difficult to detect in seed potato fields. This was not the case before 1990 when the ordinary strain (PVYO) occurred.