Symptoms and Signs
Potato Virus X
Some strains of Potato virus X produce no visible symptoms, although yields may be reduced 15 percent or more when compared to virus-free plants. Other strains cause a mild mosaic with slight leaf crinkling under periods of low light intensity and low temperature (60° to 68°F). The additional presence of Potato virus A or Y may cause severe stunting, leaf distortion, crinkling, or browning of leaf tissue.
Potato Virus A
Infection of Potato virus A appears as light yellow mottling with slight crinkling on potato plants with mild mosaic. Margins of affected leaves may be wavy, and leaves may appear slightly rugose (i.e., rough) where veins are sunken and interveinal areas are raised. Affected plants tend to open up because the stems bend outward. Severity of symptom expression depends on weather conditions, the potato cultivar, and the strain of Potato virus A. Rugose symptoms are especially evident when Potato virus A and Potato virus Y are present together.
Potato Virus Y
Potato virus Y can have the most severe impact of the mosaic viruses, depending on the virus strain and potato cultivar. Strains of PVY include PVYO (“ordinary” strain), PVYC (“common” strain), PVYNTN (tuber necrosis strain) and a recombinant strain. The symptoms will vary depending on the strain of PVY, the potato cultivar and the temperature. Also mixed infections with other potyviruses can affect the symptoms expressed. Symptoms include mottling or yellowing of leaflets, leaf crinkling, and sometimes leaf drop. Veins on the underside of leaves often show necrotic areas as black streaks. Mosaic mottling of leaves is another foliar symptom of PVY. Infected plants may be stunted. Leaf mottling may be masked at low (below 50°F) or high (above 70°F) temperatures, but at high temperatures the disease can be identified by the crinkling and rugosity of the foliage. A severe crinkling of the leaves occurs when Potato viruses Y and X occur in the same plant. The necrotic strain PVYNTN damages tubers of some cultivars, causing potato tuber necrotic ringspot disease (PTNRD). The necrotic strains of PVY exhibit only mild symptoms in potato foliage, making them difficult to manage through visual inspections of fields.
The presence of more than one of the viruses in a plant usually affects the types of symptoms and increases symptom severity. Symptoms caused by different viruses can be similar, so the type of virus usually cannot be identified by symptoms alone. Field diagnosis is often limited to calling it “mosaic virus”. Positive identification of the exact virus(es) requires the use of indicator plants, serological or DNA techniques.
Potato tubers are probably the primary overwintering reservoir of these viruses; use seed certified free from viruses or with very low incidence of infected tubers. Use resistant cultivars where possible.
Potato virus X is present in all potato varieties unless virus free material is obtained. There is no known insect vector, but the virus is carried in tubers and can be transmitted mechanically by machinery, spray equipment, root-to-root contact, sprout-to-sprout contact, or seed cutting equipment. Control is by the use of certified seed and avoiding mechanical transmission by equipment such as seed cutting machinery.
Potato viruses A and Y are carried in tubers and are transmitted from plant to plant in a non-persistent manner by several species of aphids. If a virus-carrying aphid probes a potato plant, infection is instantaneous. Insecticides applied for aphid control may not consistently prevent the spread of potyviruses, since the insecticide may not kill quickly enough to prevent transmission by winged aphids moving into the field. However, preventing aphid build up in a field may help limit virus spread. See APHIDS for more information on managing aphids.
Potato virus Y is very difficult to control due to the fact that it has a wide host range, is transmitted by over 25 different aphid species and can also be mechanically transmitted. The use of certified seed, reducing weedy areas near potato fields and preventing aphid populations from building up are critical to managing this disease.