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Vein banding caused by potato virus Y.

Potato

Mosaic Diseases Caused By Potyviruses

Pathogens:
Latent mosaic (Potato virus X), Mild mosaic (Potato virus A), and Rugose mosaic (Potato virus Y)

(Reviewed 8/07, updated 8/07)

In this Guideline:


SYMPTOMS AND SIGNS

Latent mosaic. Some strains of Potato virus X produce no visible symptoms of latent mosaic, although yields may be reduced 15% 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 crinkling, rugose mosaic, or browning of leaf tissue.

Mild mosaic. 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 mosaic. Symptoms of rugose mosaic vary widely with virus strain and potato cultivar. Potato virus Y is the most severe of the mosaic viruses. 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. 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. Some strains of Potato virus Y cause tuber necrosis symptoms.

Each virus has a number of different types or strains, and the reaction by these strains may differ. Potato variety and weather conditions also affect the symptom expression; the most pronounced symptoms appear in cloudy weather. 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 mosaic virus. Positive identification requires the use of indicator plants or serological techniques.

COMMENTS ON THE DISEASES

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. Potato viruses A and Y are carried in tubers and are transmitted from plant to plant by several species of aphids. While the virus survives only a few minutes in the aphid, if a virus-carrying aphid probes a potato plant, infection is instantaneous. Potato tubers are probably the primary overwintering reservoir of these viruses.

MANAGEMENT

Use seed certified free from viruses or with very low incidence of infected tubers. Use resistant cultivars where possible.

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Potato
UC ANR Publication 3463
Diseases
R. M. Davis, Plant Pathology, UC Davis
J. Nuñez, UC Cooperative Extension, Kern Co.
B. J. Aegerter, UC Cooperative Extension, San Joaquin Co.
Acknowledgment for contributions to the disease section:
C. Smart, Plant Pathology, UC Davis

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