Potato virus Y (PVY) is an important potato pathogen for a few reasons. Firstly, it is one of the primary factors limiting seed certification. Infected plants tend to yield significantly less than healthy plants, and some strains of PVY can cause tuber defects that affect quality.
There are several strains of PVY that occur in the U.S., including PVYO (the ordinary strain), PVYN-Wi (the N-Wilga strain), PVYNTN (the tuber necrotic or NTN strain), and others. Prior to 2010, the PVYO was the most prevalent strain in Idaho. Since that time, based on work by University of Idaho virologist Alex Karasev, we have learned that this ordinary strain has become increasingly rare in Idaho and that the most prevalent strain is now the N-Wilga strain. This strain in general causes milder foliar symptoms than the ordinary strain, but symptoms can vary from one cultivar to another. For example, in Ranger Russet, the ordinary strain of PVY induces obvious symptoms of systemic necrosis of plant tissues in the entire plant, but the foliar symptoms caused by the N-Wilga strain include moderate stunting and a mild to moderate chlorotic mosaic—which can be difficult to see, depending on timing. On the other hand, symptoms caused by the ordinary strain and the N-Wilga strain can look similar on Russet Burbank, causing a mild mosaic symptom that can be detected by a trained eye but can also be transient and difficult to see. Depending on the potato variety and the PVY strain, tuber symptoms can also appear and lead to quality problems.
PVY is transmitted from infected potato plants to healthy ones primarily by winged aphids. Aphids move PVY in a non-persistent manner, which means the aphids that vector the disease can quickly pick up the virus when they probe an infected plant, the virus sticks to the needle-like mouthpart rather than being ingested by the aphid, and when the aphid then moves to a healthy plant, it can relatively quickly insert the virus into the plant as it probes for plant fluids. The infection process can happen quickly, which is the main reason insecticides are not a stand-alone strategy for managing in-season movement of PVY. Insecticides do not kill the aphid before it inoculates a healthy plant with PVY. Although aphids are considered to be the most important way that PVY is moved from infected to healthy plants, the virus can also be moved by mechanical methods. In theory, any activity that gets infected sap into healthy plants can spread PVY.
Evidence from research conducted by a group in Canada indicates that movement of PVY by equipment and by walking through fields can occur. The same study showed that to a lesser extent, infected leaf sap that contacts cut tuber surfaces can also potentially transmit the virus. Some research conducted in the 1990s by a group in North Dakota hinted at the possibility of transmitting the virus from infected tuber sap to a healthy tuber during the cutting process, but this appeared to be a slight risk and only if the cutting tool happened to slice through an infected eye of a tuber before cutting a healthy tuber. Despite this possibility, the bulk of evidence from subsequent independent research conducted by different groups across North America over the past couple of decades show that moving PVY from an infected tuber to a healthy one by cutting blades either does not occur or is rare.
More recently, the Extension seed potato team at the University of Idaho conducted a greenhouse experiment to determine whether hand-cutting an infected tuber would lead to infection of healthy tubers cut immediately afterward. The variety used in the UI experiment was Russet Burbank, and researchers relied on naturally infected seed potato tubers produced in Idaho in an area where PVYN-Wi was the predominant strain. Results showed that PVY was not transferred by cutting an infected tuber followed by cutting healthy tubers.
In short, the available data from the scientific literature and this new research overwhelmingly support the conclusion that seed cutting does not increase incidence of PVY in a field. In the few instances where evidence suggests such movement is possible, very specific conditions must be met (such as cutting through an infected eye before cutting a healthy potato), and it occurred rarely. Growers who use cut seed can have a reasonable degree of confidence that the practice does not put their crop at risk with respect to PVY.