Symptoms on foliage

Symptoms caused by PVY vary widely with the combination of the virus strain, the potato cultivar, the environmental conditions and the type of infection (either primary i.e. current- season infection, or secondary i.e tuber-borne infection).

Field infection during the current-season with PVY^O isolates (primary infection) results in symptoms of vein necrosis and necrotic spots on the underside of the leaflets (photo 1). The leaves become brittle and dry, but remain attached to the plant (so-called leaf-drop symptom). Primary infection can also produce a distorting mosaic symptom and crinkling, often confined to one stem or one part of the plant.

Secondary infection, i.e. infection resulting from tubers contaminated from the previous year, produces more severe symptoms, which vary widely with the infected cultivar. One of the three following types can usually be observed:

• crinkling : leaf deformation (crinkling) and size reduction with shiny appearance and mottling associated with pendulous growth habit and stunting of the plants (photos 2 to 6);

• rugose mosaic : severe mosaic pattern and leaflets reduced in size, with large necrotic spots on the veins, dwarfing and severe plant deformation (photos 7 and 8);

• mosaic pattern or mottling : alternating patches of light green and dark green areas without any leaf distortion, more or less visible depending on the potato cultivar and the climate (usually more visible in overcast weather) (photos 9 to 11).

PVY^N isolates cause faint mosaic patterns in plants infected during the current season, while infection in the previous year (infected mother-tubers) gives more or less marked or mild mosaic symptoms.

PVY^N-W isolates induce faint mild mosaic symptoms (mainly in the case of primary infection) with some vein deepening (photo 12).